Australians in the nation's south-east have woken up to a steep drop in temperatures of up to 20C overnight, which have brought an end to heatwave conditions.
Several states saw maximum temperatures above 40C in the past week, contributing to life-threatening heat and destructive bushfires.
The Bureau of Meteorology (BoM) is now warning Australians of a new threat as severe storm warnings and flood risks follow the tail end of the heatwave.
Why a cool change often follows heatwaves
Residents in the east who had been sweltering under more than 40C temperatures on Saturday found relief on Sunday morning as a cool change brought the mercury down to around 20C.
In NSW, Sydney's Observatory Hill went from a top of 41.7C on Saturday afternoon to 21.6C in the space of 12 hours overnight. Residents in the town of Borrona Downs were hit by a maximum of 44.1C on Saturday, dropping to 24.7C on Sunday morning.
After a period of extended high temperatures, it's common for a cool change to follow and cause temperature drops, according to the BoM.
"This cool change swept across South Australia and Victoria on Friday, moving through New South Wales on Saturday," a BoM spokesperson told SBS News.
"The cool change sees the winds swinging around to come from the south. Winds from this direction bring in much cooler air from the Southern Ocean, sweeping away the heat."
The colder, windier conditions are now expected move through Sydney on Sunday and make their way up the north coast of NSW.
"As the cool change continues to progress northwards along the NSW coast today, fresh and gusty winds are expected about parts of the northern NSW coast," the spokesperson added.
Thunderstorms are forecast to develop through northern NSW from Sunday afternoon in the wake of the heatwave and could become "severe" around the western slopes, ranges and northern inland.
"These may bring the risk of damaging wind gusts, heavy rainfall that may lead to flash flooding, or large hailstones," the spokesperson said.
Communities should monitor for Severe Thunderstorm Warnings through this period, the BoM said.
Sydneysiders see smog from southern fires
A state disaster declaration remains in place across fire-ravaged parts of Victoria, as emergency services continue to battle out-of-control blazes.
More than a dozen fires are still burning across the state, while authorities say at least 300 buildings have been destroyed, including homes and agricultural properties.
Authorities have described the losses sustained in major fires in Walwa, Otway and Longwood as significant.
As fires continue to burn across Victoria, smog from blazes in NSW has blanketed Sydney and its surrounds.
The BoM confirmed that smoke has travelled from a fire in the state's south at Yowrie.
Tropical cyclone to make landfall
In Queensland, Tropical Cyclone Koji is expected to make landfall on Sunday near Townsville, where it will "weaken over land".
The BoM anticipates "the system may drift across northwest Queensland on Tuesday and Wednesday as a low".
A monsoon trough extends over northern Queensland and will persist for several days, which will bring scattered to widespread showers.
Premier David Crisafulli was adamant on Saturday that the already hard-hit region was "better prepared than ever" to handle the challenge.
Koji is expected to deliver heavy downpours in already-soaked catchment areas.
Despite the concern, Crisafulli said swift-water rescue craft, police, SES officers and paramedics were in position and would work with residents on the ground.
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