Lightning strikes ignited more than 250 fires in southeast Australia as firefighters battled to put out the flames and the country sweltered in a heat-wave.

Efforts in the state of Victoria on Wednesday 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 46C.

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 42C.

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

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

Western Australia has also been hit with a spate of fires. At least 44 houses have been destroyed in a forest fire on the outskirts of Perth, officials said at the weekend.

The Bureau of Meteorology said the heat-wave 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.

Last year 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.

Source: Agencies