An Ambulance Victoria spokeswoman said most of the callouts were for children who were accidentally locked in the vehicles, such as when keys were left in the car.

None of the children were seriously injured or required hospitalisation, the spokeswoman said.

The temperature almost reached the forecast 38 degrees in Melbourne, hitting a sweltering 37.8 degrees in the CBD just before 5pm. A cool change in the evening saw the mercury drop to 24 degrees by 8pm.

Firefighters had braced for hot and dry conditions during the day, with a total fire ban issued for the Mallee, Wimmera and North Central districts.

The Metropolitan Fire Brigade was called to a fast-moving grass fire near the Watergardens shopping centre in Taylors Lakes.

A southerly change fuelled the blaze, which grew to about three acres and burned six cars in the shopping centre car park just before 5.30pm.

It took more than two dozen firefighters half an hour to bring the fire under control. The cause of the blaze is yet to be determined.

A State Control Centre spokeswoman said crews responded to more than 40 grass and forest fires across Victoria, but most were small, with only about 50 hectares burned in total.

The spokeswoman said most fires were contained and there were no reports of livestock or property lost.

One fire is still burning in Minimay, in the Wimmera, with dozens of firefighters and an air tanker battling the blaze near the South Australian border.

A watch and act warning has been issued for Minimay-Frances Road in Minimay.

"Overall they've been quite small and contained, which is really good," the spokeswoman said.

"The agencies have had some really quick responses in the early stage which has helped keep those fires small."

The temperature on Tuesday was highest at Swan Hill Airport, where is reached 43.1 degrees just after 2pm, and Avalon Airport reached 39.5 degrees at 3.50pm.

Tennis Australia said the warm conditions did not have a huge impact on day two of the Australian Open. 

"Luckily, for most of the day there's been some cloud cover, which has really helped fans," said Tom Larner, director of facilities and events at Tennis Australia.

"Thankfully, the top temperature has come later in the day, when there's not as much direct sunlight."

Mr Larner said crowd numbers were down from Monday, when a crowd of 72,424 attended - a record for the first Monday of the tournament. By 5pm on Tuesday, 37,789 attended the tennis.