Emergency-level bushfires burned across Victoria on the state's hottest December day on record, with seven people being treated for injuries or heat stress.

Key points: Several towns and cities in the state's north-west have broken December heat records

Several towns and cities in the state's north-west have broken December heat records Residents near an emergency-level grassfire in Victoria's west were told it was too late to leave

Residents near an emergency-level grassfire in Victoria's west were told it was too late to leave For more warnings and information, visit the VicEmergency website

Five properties were damaged and residents described hearing massive explosions and seeing flames and shrapnel flying into the air as gas cylinders exploded during a fire in the Melbourne suburb of Seaford.

The ensuing grass fire threatened a petrol station on the Frankston-Dandenong Road and the old Nylex factory.

The Country Fire Authority (CFA) said five firefighters received heat-related injuries. One of the injured firefighters was treated for smoke inhalation after helping to evacuate a childcare centre in Seaford. As well as the injured firefighters, one person was treated for minor burns and another for shock.

Several properties were badly damaged at Seaford. ( ABC News: Zalika Rizmal )

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On Friday night people in the township of Lexton, in the state's west, were advised to evacuate as two emergency warnings were issued for a fire moving east towards the town from Mount Lonarch.

That recommendation to evacuate was lifted early on Saturday morning but nearby residents were still urged to leave in case conditions worsen.

New fires were also cropping up along the border with South Australia as a band of lightning crossed the state.

Residents near fires burning at Dunkeld, north-east of Hamilton, and at Brookville, north of Bairnsdale, were told conditions could worsen quickly and leaving was the safest option.

The fire at Seaford damaged buildings and vehicles. ( ABC News )

The Seaford fire was eventually downgraded from a watch and act to an advice message.

But earlier firefighters called in a water bomber to tackle the blaze as spot fires broke out up to a kilometre ahead of the main fire at Webb Street.

The house fire at Seaford spread to adjacent bushland. ( Facebook: Chantelle Beattie )

A total fire ban has been declared for all of Victoria and watch and act messages were also issued for fires burning in the state's south-east and south-west.

December temperature records tumble

Hopetoun, south of Mildura, broke the record for Victoria's hottest December day when the mercury hit 47.4C at 2:16pm.

The town reached 47.9C during the peak of the heat.

The previous December record was 46.6 degrees at Robinvale in 1976.

That record was today broken at 10 different centres including Horsham, where the mercury hit 47.9C — that city's hottest-ever temperature.

Other record-breaking temperatures were recorded at Walpeup, 47.7C at 3:32pm and Mildura which hit 46.8C at 3:49pm.

Sandra Bone is recovering from a fractured hip, but still hit the beach in Ocean Grove. "The sea has special healing properties," she said. ( ABC News: Nicole Mills )

These emus at Halls Gap were also feeling the heat. ( Supplied: Andrew Pakenham )

In central Melbourne, the temperature reached 43.5C, just shy of the 43.7 degree record set in 1876. At Melbourne Airport, it hit 44C.

Avalon hit 45.8C at 4:07pm and at Laverton the top temperature was 45.2C at 5:36pm.

A cool change started to sweep across the state in the evening, with the temperature at Portland dropping from 43.6 degrees to 20 degrees.

The weather bureau said the weekend would be much milder in the south and in the north, where temperatures would hover in the 30s.

Phil O'Day has been battling drought and record-breaking heat on his farm in Victoria's Millewa region. ( ABC Mildura-Swan Hill: Jennifer Douglas )

Trina Bellinger, who owns the Hopetoun Community Hotel, said the town was coping well with the oppressive heat.

"We live in the Mallee so you've got to be used to the heat," Ms Bellinger said.

Local resident Suzie Hallam said she had been taking reprieve in the air-conditioning at her accommodation park on Lake Lascelles in Hopetoun.

She said the heat was "sucking the moisture from everything" around her.

"Trees that were beautiful and green this morning are just crumbling," she said.

Victoria's emergency management commissioner, Andrew Crisp, said the cool change would take some time to move across the state.

He said the cooler weather would come with its own danger.

"We know there is a potential for dry lightning and fires as that change moves across the state into tonight and the early hours of the morning," Mr Crisp said.

Smoke shrouds sweltering Melbourne

Melbourne's CBD can be seen covered in smoke from South Kensington train station. ( ABC News: Elise Kinsella )

Melbourne was affected by smoke from both NSW and Gippsland. ( ABC News: Melissa Brown )

Smoke from fires burning in NSW as well as Victoria, reduced visibility to about a kilometre in some parts of the state.

The EPA's chief environmental scientist Dr Andea Hinwood said there had been extremely poor air quality in Melbourne, the Latrobe Valley and parts of the north-east of Victoria, because of smoke from fires.

"Up in the Mildura area we know we have very high concentrations of dust, we also have some high dust levels around the Melbourne metropolitan area and also in the Latrobe valley," she said.

A severe thunderstorm warning has been issued for parts of the Mallee, and Wimmera forecast districts in the far west of the state.

The Bureau of Meteorology says severe thunderstorms may produce damaging winds in the area but the storms will bring little if any rain.

There is also the potential for dust storms caused by wind gusts which may affect areas including Horsham, Edenhope, Warrnambool and Portland.

Power companies are also working to restore power to more than 17,000 homes and business across Victoria.

The majority of customers without power are United Energy customers in suburbs between Ringwood and Portsea.

There were a number of delays on Melbourne's train network due to the heat.

Metro Trains imposes heat restrictions when the track temperature exceeds 55C, slowing trains down.

There were major delays on the Frankston, Cranbourne, Mernda and Sunbury lines at 6:00pm and minor delays on a number of other lines.