Ambulances and hospitals struggle to cope with massively increased patient numbers.
The sweltering conditions are more deadly than a bushfire, claiming hundreds of lives.
This isn't some dystopia. It already happened, in 2009.
And many fear Melbourne might experience such extreme conditions again this summer.
How would the city cope? What is being done to plan for such a weather event? How well prepared are we?
Hot in the city
Melbourne is set for a hot summer of warmer than average days and nights. Photo: Leigh Henningham
After a week of sweltering days and balmy nights, Melburnians have already put in some serious practise for a long, hot summer.
Last week's spell of six consecutive days above 30 degrees was the longest in November since official records began in 1862.
Melbourne posted 12 days above 30 this spring, well above the seasonal average of just two. The earliest was in September.
It's a taste of what's to come.
The Bureau of Meteorology's latest climate update shows Victoria is likely to experience "warmer than average" temperatures over summer, both during the day and overnight.
The forecast is also for an "early and above average" bushfire season after a very dry spring. The first total fire ban of the season, in the Mallee on October 12, was the earliest issued in Victoria.
"It's not the same backdrop as 2009, but that's not to say we won't have days or weeks where there are significant weather events that could see us quite challenged by fire or heat," said Victoria's Emergency Management Commissioner Craig Lapsley.
"I think the forecast now is we have to be as much focused on heat as on bushfires, in that it's really clear ... above-average temperatures both day and night."
A bushfire near Healesville on Black Saturday. Photo: Rob Carew
Mr Lapsley is keenly aware of the dangers of prolonged hot spells. "The global figures tell you more people die from heat than from bushfires," he said. "It's a reality.
"Obviously in urban areas, you're not threatened by bushfires if you live in Brunswick. But in cities and urban centres, heat is retained overnight, so our asphalt and our concrete environments hold heat."
Emergency Management Victoria has to plan for what happens when everything goes wrong.
"We work on energy disruption plans, we've got plans around public transport disruption ... We plan from the [scenario] that these events are likely to happen ...
"And the day they're likely to happen, they're not in isolation. It'll be the hottest, windiest day; it will be the day there's smoke in the sky; people will be fatigued."
University of Melbourne climate scientist Andrew King said the likelihood of extreme heat this summer was difficult to accurately predict. A looming La Nina weather pattern complicates matters.
"During La Nina events most of eastern Australia is a bit cooler through summer, Dr King said. "But Melbourne is a bit different. We sometimes get hotter heatwaves during La Nina at times.
"There's a lot of difference between summers ... [but] there's a decent chance we will get heatwaves of over 40.
"With La Nina events, if it does come to pass, it's likely we would have fewer of the hot days, in the 30s, but an increased chance of extreme heat, in the 40s."
On the rails
Trains have to slow down in the heat. Photo: Joe Armao
Upgrades to Comeng trains, the wider use of concrete rather than timber sleepers, continuous welded rail, and temperature sensors on tracks have helped Melbourne's trains run more smoothly during hot periods in recent years.
Metro Trains claims the system is now well equipped to handle Melbourne's hot weather, but if temperatures reach 42 degrees, trains across the network will be forced to slow to 70km/h.
Unexpected temperature changes may also result in speed reductions to prevent the steel tracks from expanding.
Commuters had a taste of this on the hottest day so far in November, when trains on the busy Frankston line slowed to 80km/h on the 34-degree day. (Trains usually run at a 110 km/h speed limit, reaching a maximum of 150km/h in some sections.)
But trains are not the only form of public transport vulnerable to the hot weather. Trams on a quarter of Melbourne's routes still have no air conditioning. This includes Route 57 and 82 (Z-Class trams), Routes 12, 30 and 78 (A-Class trams) and Route 35 (W-Class).
Public Transport Users Association spokesman Tony Morton said air conditioning on trams was important, but so too were windows that can be opened, of which there were few on newer E-Class trams. (Windows on C and D-class trams can't be opened, so they are removed from service on hot days).
"If the air conditioning dies, these trams are more uncomfortable than trams which do not have air conditioning but have all the windows open."
When trams and trains fail, commuters rely on the neglected bus network to pick up the slack. Bus companies are contracted by Metro Trains and Yarra Trams to fill gaps during planned and unplanned cancellations.
But what happens when the buses shut down in the warm weather too? On November 14, the Frankston train speeds were reduced, 38 Transdev buses were cancelled in the afternoon peak due to heat, leaving passengers stranded in the CBD.
Engines broke down on several A66 and A76 MAN buses, models which make up 62 of Transdev's 510 buses in Melbourne.
"If these summer heatwaves are going to be more common in the future, we can't have essential infrastructure breaking down every other week," Dr Morton said.
An extra 2000 megawatts have been found to make up for the closure of Hazelwood. Photo: Bloomberg
Victoria looks likely to make it through summer without experiencing damaging blackouts despite fears over many months that the closure of the Hazelwood power station would leave the state dangerously short of energy on hot days.
The 53-year old coal-fired clunker was the dirtiest power station in Australia, but it also supplied more than 20 per cent of the state's energy capacity and its mothballing stripped 1600 megawatts from the grid.
The Australian Energy Market Operator sounded the alarm bell in September when it released a report warning of "an increased risk of energy shortfall over the next 10 years".
The report put Victoria right in the crosshairs of the nation's energy reserve shortage, predicting the state had a 39 to 43 per cent risk of shortfalls this summer.
The sense of urgency was heightened this month by the failures of two generating units at the Loy Yang and Yallourn power stations, which temporarily drained a further 1300 megawatts of power from the state grid.
These failures also pointed to the prospect of Victoria's coal-fired power plants becoming increasingly unreliable as they age, a charge renewable energy sceptics like to level at solar and wind.
But by this week AEMO was telling everybody to cool it. It had sourced an extra 2000 megawatts of capacity, more than enough to make up for Hazelwood's demise and let everyone keep the aircon running on a 40-degree day.
The Andrews government, meanwhile, is betting on renewables to power Victoria in the years ahead.
It has set a target of 25 per cent renewable energy generation by 2020, including 650 megawatts of wind and solar to be delivered by the private sector in a series of "reverse auctions".
But one piece of the puzzle - 100 megawatts of dispatchable battery storage in western Victoria that was promised to be ready by January - appears to have fallen flat.
Instead a similar amount of back-up power will be supplied by diesel generators, a far less environmentally friendly energy source.
Fire on the fringe
CFA truck at fire in Bunyip State Park. Photo: William West
Despite its rural name, Victoria's Country Fire Authority covers 60 per of greater Melbourne.
CFA chief officer Steve Warrington said many people who moved to major growth corridors on the urban fringe had little idea of the bushfire risk in their neighbourhood.
Last month's blazes in northern California showed how urbanised residential areas could be "wiped out because of bushfires," he said.
Close to 9000 buildings were destroyed, 43 lives were lost, 185 people were injured and about 90,000 evacuated after fires burnt across more than 210,000 hectares in what became the deadliest week of wildfires in California history.
Areas of greatest concern in greater Melbourne this summer include those that were hit hard in 1983's deadly Ash Wednesday bushfires.
"That took out Upper Beaconsfield, Cockatoo, Gembrook, that whole area of the Dandenong Ranges," where there are long-established neighbourhoods but also growth areas.
"There is a whole heap of population that has gone into those areas that are new. They may not even be aware that there was a fire in that area that was quite damaging."
Mr Warrington said tracts of grassland on the western plains, which abut developments in relatively new suburbs, also posed a significant bushfire risk.
"A place like Truganina, if you go back a couple of years that was all flat grasslands. Now we've got grasslands going up to houses."
The CFA has hired more professional staff in some high-risk areas, improved the design and safety of its trucks and introduced extra training and education campaigns since Black Saturday.
This summer it has boosted its aerial fleet, with an extra helicopter stationed at Bacchus Marsh, and will send aircraft out sooner to douse fires before they get out of hand.
The biggest and best method of fire prevention was getting people to recognise the risk of fire and plan how to respond, Mr Warrington said.
It is a sobering reality that firefighters cannot get to every emergency. "We lose more homes than we have fire trucks," he said.
"The message for the community is that we'll do whatever we can, but it is about making a plan. It could be a bushfire, a grass fire. They could be at home or travelling."
For heat events, the most important and effective strategy was for individuals to make plans well before a heat event, Mr Lapsley said, for people in their household as well as animals.
"If people have companion pets, hopefully people are doing the right thing, building their pets into their plan," he said, particularly ensuring that those left at home in the heat of the day have access to water and cool or shady areas.
"Think about how children will get home from school if the trains or trams aren't running, or if the power is out and you can't recharge your phone.
"Take the two minutes to think about the things you use every day: if they're not available, what would you do?"
with Adam Carey and Timna Jacks