A blast of heat with tropical conditions to follow has authorities in several states issuing alerts.
Most serious is the rises in mosquito-borne virus detections in recent days.
There are a rising number of either detections or human cases of Ross River Virus in Western Australia, Victoria, New South Wales and Queensland.
South Australian health officials are warning about the risks of Murray Valley Encephalitis and the Kunjin virus after the virus was detected during routine monitoring.
Potentially toxic blue green algae blooms continue to spread in many Victorian waterways and particularly in the Murray River.
Red alert warnings have been issued on the latest bloom which now stretches from Red Cliffs to Lock 9, above Wentworth and including Mildura.
There is a fear more people plan to go camping on the final weekend of the school holidays exposing themselves to greater risk.
The Bureau of Meteorology predicts temperatures will soar to 45 degrees at Mildura in Victoria on the weekend.
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Similar extreme temperatures are forecast for southern NSW on the weekend with Deniliquin forecast to reach 45 degrees on Monday.
Longer range forecast also predict the tropical cyclone is expected the hit the north-west coast of Western Australia on Friday.
This system will degrade to a tropical depression but continue to spiral south east through WA and impact on the weather of South Australia, NSW and Victoria later next week.
Heat and humid conditions are ripe for the growth of blue green algae.
Health authorities saying covering up at peak mosquito times and the use of repellents is the best defence against mosquito bites.