Lightning strikes ignited more than 250 fires across southeast Australia overnight, officials said Wednesday, as firefighters battled to put out the flames and the country sweltered under a heatwave.

In Victoria state, where catastrophic firestorms claimed 173 lives and razed more than 2,000 homes in 2009, efforts focused on containing about 20 blazes.

A further 18 fires had already been brought under control or were being mopped up, and officials said no homes were currently under direct threat.

"Firefighters kept busy with 256 new fires started by lightning overnight," Victoria's Country Fire Authority said.

Western Australia, South Australia and Victoria have in recent days sweltered under temperatures reaching 46 degrees Celsius (115 Fahrenheit).

Tennis players wilted in the conditions at the Australian Open in Victoria's capital Melbourne, with some vomiting and blacking out as the thermometer hit 42 degrees Celsius.

In neighbouring South Australia, the capital Adelaide sweated through its fourth hottest day on record Tuesday, with a high of 45.1 degrees Celsius, and was again experiencing searing heat on Wednesday.

Firefighters faced horrendous conditions after hundreds of fires were sparked by lightning in the state on Tuesday, with more than a dozen blazes still burning Wednesday and aircraft water-bombing the worst-hit areas.

One woman was in hospital with non-life threatening burns to her face and chest after struggling to save her house at Rockleigh, south west of Adelaide.

"The house just caught (fire) while she was in there and her husband came running to the rescue and ripped her out while she was trying to grab some small animals," her brother Mark Hansen told the broadcaster ABC.

The Bureau of Meteorology said the heatwave was focused mainly over southeastern Australia and had substantially subsided in the west, except for the northern areas including the resource-rich Pilbara region.

But severe conditions are forecast for southeastern South Australia, southern New South Wales, Victoria and Tasmania.

Officials in Victoria are concerned about an expected increase in winds on Friday that could spread flames.

2013 was Australia's warmest year since records began in 1910, and 2014 has begun with scorching conditions with more than 50 homes destroyed in bushfires in the western city of Perth on Monday.