Power has been restored to nearly all Victorian customers affected by rolling blackouts, after Melbourne's hottest day in five years placed "unanticipated levels of demand" on the grid.

Key points: Parts of Melbourne and regional Victoria lost power due to rolling outages

Parts of Melbourne and regional Victoria lost power due to rolling outages A cool change in Melbourne sent temperatures plummeting from above 40C to below 30C

A cool change in Melbourne sent temperatures plummeting from above 40C to below 30C Firefighters are battling an out-of-control blaze threatening homes in remote east Gippsland

The Australian Energy Market Operator (AEMO) ordered load shedding across the grid earlier Friday in order to deal with the increased demand.

Load shedding occurs when demand is too great, so the energy operator directs power companies to create rolling blackouts in different parts of the network so the entire system is not at risk.

Just after midday, AEMO chief executive officer Audrey Zibelman said the power outages would affect about 60,000 customers for up to two hours.

Loading

AEMO later confirmed that more than 200,000 Victorian consumers had been impacted by the load shedding over the course of the day.

"AEMO acknowledges the inconvenience and hardship of losing power supply during extreme conditions, and thanks all Victorians for their patience," it said in a statement.

It also thanked consumers who had "purposefully conserved their energy use over the past 48 hours".

Energy Minister Lily D'Ambrosio said "unanticipated levels of demand" had led to the outages.

She said most of the loss of capacity was due to three coal-powered generation units in the Latrobe Valley being offline, in combination with a reduction in output from the remaining units.

"We know our summers are getting longer, they're getting hotter and they're getting more extreme because of climate change," she said.

"We have a 20th century energy system for a 21st century climate."

Ms D'Ambrosio had earlier said the Government was not anticipating load shedding to occur, despite the huge demand for power.

Opposition Leader Michael O'Brien said the state was "not a third world country" and the blackouts were unacceptable.

"Something is seriously wrong when the power goes out in Victoria because we don't have enough supply," he said in a statement.

"When Labor policy led to the closure of Hazelwood power station, the Liberals and Nationals warned that Victoria was left exposed."

Cool change arrives in Melbourne

A day of intense heat saw the mercury hit 46C at Melbourne Airport, making it the hottest on record for the area.

But relief came to Melbourne early Friday afternoon, as a cool change swept in from the west.

When it hit the city at 2:00pm, it took just 10 minutes for the temperature to drop from 42.3C to 29.9C.

And with the cool change came the return of power, with energy companies directed to start restoring electricity to thousands of Victorians.

As of 4:30pm, the number of properties without power had dropped to around 11,000.

Earlier in the day, AEMO activated emergency reserves, including preparing to pay some businesses to voluntarily curtail their own usage.

But those reserves were not enough to cover the shortfall.

Power was also imported from South Australia, Tasmania and New South Wales, but more could not be brought in as interconnectors were at their maximum limits.

One company sent customers a text message stating that a "PowerResponse event" would occur between midday and 3:00pm "due to extreme weather".

Elwood Beach was already popular with swimmers about 7:15am as the mercury rose above 32C. ( ABC News: Natasha Johnson )

'It's gone dark'

On Ormond Road, the main street of Elwood in Melbourne's inner south, shops and cafes were forced to limit services or close because of the lack of electricity.

"It's gone dark all the way along the main street here," Elwood cafe worker Sofie Godefrooij said.

"It's all pretty quiet because we can't make coffee or serve much food now."

Regions hit by outages include: Epping, Mill Park, Lysterfield, Rowville, Clyde, Cranbourne East; Southbank, Armadale, Toorak, Camberwell, Fairfield, Northcote, Caulfield, Elwood, Beaumaris, Bulleen, Burwood, Riversdale, Bentleigh, Heathdale, Balaclava, Malvern, Balwyn, Surrey Hills, Essendon, Essendon North, Strathmore, Glenroy, Oak Park, Kalkallo, Broadmeadows, West Footscray, Fairfield, Ivanhoe, Alphington, Airport West, East Keilor, Niddrie, Ascot Vale, and Moonee Pond, Camperdown, Weerite, Yarrawonga, Bundalong, Castlemaine Muckleford, Lara, Corio, Norlane, Herne Hill, Hamlyn Heights, Balmoral, Haven, Woolsthorpe, Grassmere and Mailers Flat. Source: AEMO

The Malthouse Theatre and Australian Centre for Contemporary Art (ACCA) in Southbank were hit with a power outage about 12:30pm, forcing the cancellation of an evening show at the theatre, the early closure of the theatre's cafe and the temporary closure of ACCA.

Producer Jason Tamiru's free show, Blak Friday, was set to launch the theatre's Civil Rights Summer Series of events.

"It was a celebration of everything black," he said.

"The theme was black Friday, and unfortunately it's turned out to be blackout Friday. It's unfortunate but what are you going to do?"

Cafe owner Murray Pitman closed up early, with the violent heat slowly spoiling the food packed in his fridges and cool room.

"Obviously we've got quite a bit of perishable food that … it's no longer safe to sell," he said.

"Friday lunchtime, no it's not ideal. But you roll with the punches I guess."

More than 100 sets of traffic lights also went out across the city, and at 3:30pm Victoria's traffic authority warned motorists to take care as a number of traffic lights and signs remained without power.

Construction ground to a halt in parts of the state, after hundreds of workers on unionised building sites downed tools in the heat.

The CFMEU, which represents construction workers, has a heat policy that states work must stop once the temperature hits 35C.

More than 160 tower cranes in Melbourne alone have been shut down, according to the union.

Elwood office workers left work early after power was cut in the suburb, to enjoy a beer in a dark cafe in the main street. ( Supplied: Sofie Godefrooij )

In Shepparton, in the state's north-east, the manager of the Sherbourne Terrace Hotel, Nick Zurcas, said the power went off midway through the lunch rush, causing the hotel to lose customers.

"[We're] struggling big time, unfortunately on one of the hottest days of the year," he said.

"We had to go around and ask customers could they change their orders because we couldn't cook certain items."

He said he was not surprised by the situation.

"Surprised no, disappointed yes. Everyone knew what was going on — it's embarrassing for Australia to be in this situation," he said

Bar manager Nick Zurcas said the power outage hit midway through the lunch rush. ( ABC News: Mahalia Dobson )

Ms Zibelman said Alcoa had agreed to power down its aluminium smelter at Portland, in the state's south-west, to ease demand, as it had the previous night.

"They've been extraordinarily helpful to us and they've agreed to go off today too," Ms Zibelman said.

Separately — and unrelated to AEMO's load-shedding directions —various local outages throughout the state left more than 5,000 households without power for part of Thursday night, while a fault with an underground line left about 2,700 houses without power in central Victoria on Thursday.

Bushfire threatens homes in state's east

In the east of the state, fire crews continued to battle an out-of-control bushfire threatening homes.

An emergency warning remained in place for Butchers Ridge, Gelantipy and W Tree late Friday night.

The fire has been burning for a number of days, but Friday's conditions saw it grow from around 400 hectares in size that morning to around 3,000 hectares by mid-afternoon.

Emergency management commissioner Andrew Crisp said up to nine water bombers had been used to attack the fire, though crews on the ground had to be withdrawn due to the conditions.

"It is a dangerous fire, so we've had to move firefighters out of that particular area for a period of time until things settle down," he said.

A relief centre has been set up at the Buchan Recreation Reserve and authorities warned any residents at nearby Gillingall to leave their homes before conditions become too dangerous.

"Emergency services may not be able to help you if you decide to stay," the warning said.

W Tree resident Kay Schieren stopped in at the relief centre in Buchan earlier Friday as he made his way towards the safety of the coast.

"Tonight we are probably moseying down to a friend's place in Bruthen or Metung to stay the night, and hopefully go home tomorrow and see what it looks like."

W Tree resident Kay Schieren is spending the night by the coast. ( ABC News: Emma Field )

He said conditions were very hot, windy and smoky when he left W Tree earlier Friday, and he was keeping a cautious eye on the cool change, which was expected to reach the area before midnight.

"I hope that that's not accompanied by too many lightning strikes," he said.

The Timbarra fire was one of around 40 that crews were dealing with on what the commissioner described as a "fire spike day".

"Our firefighters on the ground and in the air have done a fantastic job," he said.

Commissioner Crisp earlier warned that Friday's extreme weather meant that any fires that did spark would be "uncontrollable".

"They will be fast-moving … I believe we're well and truly ready as a sector. The question is, members of the public, are you ready?"

He said 500 emergency management staff were in place across the state, and 60 planes and helicopters were poised to tackle fires.

He urged people not to leave children in locked cars in the heat and to download the VicEmergency phone app, which can be tailored to the user's location and provides information on fires, floods and other emergency incidents, to stay informed.

Emergency management commissioner Andrew Crisp warned extreme weather would make any fires difficult to control. ( Facebook: Wodonga West Fire Brigade )

Australia Day concerns

A total fire ban was in place across the entire state on Friday, with the fire danger rated extreme for the North Central, Northern Country and Mallee districts, and severe in most other areas.

Authorities were also concerned careless visitors over the Australia Day long weekend could put extra pressure on fire crews, with concerns around campfires and fireworks.

"We've really hit a critical point in the bushfire season," said Aaron Kennedy, the Department of Environment, Land, Water and Planning's assistant chief fire officer for the Hume Region.

"So we're really asking people to be vigilant, particularly if they're in our forests or on the river this Australia Day weekend.

"Any fire that starts, we're seeing it's going to be difficult to control."