Residents have been urged to take shelter indoors as a bushfire threatens homes and lives south of Rosedale, in Victoria's Gippsland region.

Key points: A bushfire near Rosedale in Gippsland burnt more than 550 hectares

A bushfire near Rosedale in Gippsland burnt more than 550 hectares Another fire was burning at Strathbogie in the state's north-east

Another fire was burning at Strathbogie in the state's north-east Temperatures exceeded 40 degrees in many parts of the state

Authorities issued an emergency warning shortly before 10:00pm on Friday for people in Fulham, Kilmany, Kilmany South and Pearsondale.

"You are in danger and need to act immediately to survive," the warning said.

"The safest option is to take shelter indoors immediately. It is too late to leave."

Thick smoke from the fire darkened the sky over Sale on Friday evening. ( ABC News: Mark Debono )

Firefighters were using 12 aircraft including two large airtankers and 26 fire trucks to fight the blaze, which had burned through more than 550 hectares.

A wind change caused the fire to change direction on Friday night, and it was burning in a north-easterly direction towards the Latrobe River and the Princes Highway.

An emergency warning issued for residents in the Stradbroke West, Willung and Willung South shortly before 5:30pm was downgraded to a Watch and Act message about 7:45pm.

The CFA and Victoria Police will investigate the cause of the fire.

A Watch and Act alert was also issued for Strathbogie in the state's north-east, where a grassfire was burning out of control.

Authorities urged residents to leave, warning conditions could get worse quickly.

Firefighters were using 11 aircraft and 30 fire trucks to battle the blaze, which was travelling in a south-easterly direction towards North Creek Road in Merton.

"We've hit it very hard once it was up and running," Mr Cook said.

"I think we've got that under 100 hectares at the moment."

The fire near Rosedale burned through more than 550 hectares. ( Supplied: Gaynor Rettino )

"That fire will burn under reasonably consistent conditions until dark and our crews on the ground and also the air crews are working really hard to try and get that one stitched up before then," he said.

"Fingers crossed that they'll be able to do that."

For the first time in Victoria, firefighters were planning to use helicopters with night vision equipment to waterbomb the Strathbogie blaze through the night.

'Safe to touch ... safe to leave'

Unattended camp fires sparked a series of blazes across Victoria on Friday, infuriating fire authorities, who had declared a total fire ban for the entire state and issued extreme fire danger warnings for the Mallee Wimmera, Northern Country and North Central districts.

"This behaviour has the potential to create such a significant impact," said Emergency Management Commissioner Andrew Crisp.

"It really is dumb.

"It should not happen and I guess the message we want people to take out of this is you've got to test that fire.

"If it's safe to touch, it's safe to leave."

Victoria has already endured more than 700 fires since the start of December.

Emergency services have already battled more than 700 fires this summer. ( AAP/Supplied: CFA, file photo )

Temperatures plunge after scorcher

Temperatures in many parts of the state exceeded 40 degrees.

The town of Walpeup, near the South Australian border, reached 46 degrees, as did Hopetoun. Melbourne reached 42 degrees.

A cool change travelled east across the state over the course of the day, reaching Melbourne shortly after 4:00pm.

In Melbourne the mercury plunged from 42 to 21 degrees in the space of an hour.

Earlier, temperatures in Geelong dropped from 44 degrees to 31 degrees in 15 minutes.

Melburnians flocked to swimming pools and beaches to beat the heat on Friday. ( ABC News )

Much milder conditions were expected over the next few days, according to Richard Russell, a senior forecaster at the Bureau of Meteorology.

"From going from 15 degrees or so above average temperatures across the state today, southern Victoria can expect temperatures around five degrees below average [on Saturday]," Mr Russell said.

"So very typical of Victoria really."

Children still being left in cars

However, authorities were dismayed at the number of children being left in cars, despite numerous warnings this summer.

Emergency services responded to at least 10 instances today, including two children left in a vehicle in Melbourne's south-eastern suburbs.

Ambulance Victoria's Justin Dunlop, who is also the State Controller for Heat, urged Victorians not to become complacent.

"It's never okay to leave a kid, an animal, pet or anyone in a vehicle particularly on a hot day like today," Mr Dunlop said.

"Make sure you look before you lock in case you leave someone behind."

The temperature rose to 42 degrees in Melbourne before dropping sharply after 4:00pm. ( ABC News )

The Emergency Services Minister, Lisa Neville, also pleaded with Victorians to be careful around water, with a spate of recent drownings.

At beaches across the state, 37 people were rescued today, while four boats capsized on Victorian waters.

"Be aware of the conditions," Ms Neville said.

"Take care of yourself. Make sure you don't swim beyond your capability. If you're in a waterway, make sure you understand what's underneath that water," she said.

"We do not want to see another drowning death in Victoria."

Seven people have died in Victorian waters since Christmas Eve.

Meanwhile, residents in parts of north-east Victoria were urged to restrict their water use to essential needs after two pumps failed at the Seymour water treatment plant.

Goulburn Valley Water said its staff were working on the issue.