South-eastern Australia faced some of the worst fire conditions for five years after a week of scorching temperatures set bushfires raging across swathes of the region and threatened to cause power outages on Thursday.

Tourists were told to leave the popular Grampian national park in Victoria as two out-of-control bushfires merged to form a 1,500-hectare blaze bearing down on rural communities around the park's northwest entrance.

Firefighters expect strong winds to keep the fire complex burning into Friday.

The blaze was one of several causing concern on Thursday night after a third day of severe temperatures, which brought a range of problems:

• Matches were suspended on the outside courts at the Australian Open as Melbourne recorded its highest temperature - 43.9C - since Black Saturday in 2009.

• Adelaide was declared the hottest city in the world by the United Nations' World Meteorological Organisation, with the temperature hitting 44.2C

• High electricity use and generation plant outages put a major strain on electricity supplies, with Victorians using almost twice the average amount of power for a January weekday.

• Residents were urged to leave the Victorian towns of Tempy and Gypsum in the north-western part of the state. Emergency warnings were also issued for fires at Trafalgar, in the state's east, and Telopea Downs on the South Australian border.

• Fires were also burning across parts of South Australia with crews battling fires on the west coast, Eyre Peninsula, the mid-north, the southeast and the Riverland.

Victoria's Fire Services Commissioner Craig Lapsley said predictions for Friday for 40C-plus temperatures, strong winds and a late gusty change made for some of the state's worst conditions since 2009.

"It's certainly up there - the fact that we've had a pre-run of three, four days of this sort of weather," he said.

A total fire ban has been declared for the entire state on Friday.

Crews were kept busy by 1000 new incidents on Thursday, the majority of which were small fires sparked by lightning strikes.

Victoria's department of primary industries issued an emergency warning on Thursday afternoon, saying a fire heading in a southerly direction from Bronzewing could hit Tempy and Gypsum between 4pm and 6pm (AEDT). Residents were advised to leave their homes and head to safer areas.

An emergency warning was also issued for Roses Gap, Heathvale, Laharum, Zumsteins and Wartook in the Grampians region, which lies west of Melbourne.

In Melbourne, Australian Open organisers enacted the tournament’s extreme heat policy, following several days of complaints from some players over the conditions.

The roofs at the two show courts – the Rod Laver Arena and Hisense Arena – were closed, while play was suspended until 5pm on the outside courts.

Adelaide was predicted to break its record temperature of 46.1C set in 1939 but the mercury peaked at 44.2C in the late afternoon.

Friday is also expected to be very hot with the Bureau of Meteorology forecasting a top of 42C before a cool change sweeps in dropping the likely maximum on Saturday to 28C.

The five days at 40C or above will make it the city's third worst heatwave on record.

Large fans with water mist sprayers were set up at the Rundle Mall. The council said it is advising people to drink plenty of water, dress in cotton lightweight fabrics, avoid going out in the heat and use air conditioning and fans if possible.

The hottest spot in SA on Thursday was Roseworthy, just north of Adelaide, which had a top of 46.4C.

Adelaide is set to endure five consecutive days over 40C, while Melbourne is on course to have four days above this temperature, the first time this has happened since 1908. The Victorian capital is forecast to reach 44C on Thursday.

In the rest of the state, 18 fires were causing concern with the worst of those near Port Germein, north of Adelaide, where a blaze in largely inaccessible scrub is a threat to lives and homes.

The Country Fire Service said the blaze was burning out of control, prompting an emergency warning with locals urged to activate their bushfire survival plans.

Water bombing aircraft have been deployed with ground crews ready to attack the fire when it moves onto lower ground.

"It's been burning for a couple of days in very steep country and has been giving us some headaches," a CFS spokesman said.

"We will have to work very hard tonight."

Victoria's energy and resources minister Nicholas Kotsiras said forced blackouts were possible as the high temperatures put the national electricity network under serious strain.

Victorian demand has topped 10,000 megawatts for three consecutive days, compared to the 6,600MW average for a January weekday.

The Australian Energy Market Operator (AEMO) was again considering reducing supply to Victoria amid record high electricity demand, which could affect 100,000 homes and businesses.

Premier Denis Napthine said the problem was exacerbated on Wednesday by one of the four generators at Loy Yang A power station breaking down, and the Basslink cable between the mainland and Tasmania not operating at full capacity for technical reasons.

Mr Kotsiras said the total supply available to Victoria improved on Thursday, with the Loy Yang unit back to partial service.

The AEMO says there should be more than 1,000MW of reserve on Friday in Victoria.