The Harbour City is 17 days into its stretch of 20-degree or warmer days, with seven more days of such weather possible, said Brett Dutschke, senior meteorologist for Weatherzone.

Only once in 150 years of records – in 1987 – has the city had such warm conditions lasting this long this late in the year, Dr Dutschke said.

Many other regions have been experiencing unseasonably warm weather this month, with more to come. Melbourne, for instance, can expect five days of 20 degrees or warmer days, starting Wednesday.

Adelaide, meanwhile, may get five days of 25 degrees or hotter conditions starting today, a spell not seen this late in a year since 1921, Dr Dutschke said.

Australia has experienced a string of heatwaves, roughly six weeks apart, for the past half-year or longer, climate experts at the Bureau of Meteorology say.

Those hot spells produced the hottest month on record, the hottest summer and a blitz of other national heat records.

Five of the bureau's 112 long-term weather sites have already registered all-time May records, with towns such as Alice Springs in the middle of what forecasters expect will be the longest run of 30-degree or hotter days.

“It's a lack of strong cold frontal systems pushing cold air into the continent,” Blair Trewin, senior climatologist at the Bureau of Meteorology, said. “Most of the fronts that are occurring are being deflected south of the continent.

“We are clearly going to be a long, long way above average [for the month], nationally, as of the 11th to 12th of May.”

In addition to the warmth, “huge swathes of the country have had no rain this month”, he said.

A third of Queensland has already been declared in drought conditions and many parts of south-eastern Australia are recording rainfall far below average for the crucial crop-planting season.

The records to fall include some beyond the shores. Sea-surface temperatures along almost all of the southern coastal, for instance, and around most of Tasmania were the highest on record in the first four months of 2013, the weather bureau said.

Storm front

The warm conditions may finally break up in the next week or so but the weather system packing such a punch will also hammer much of south-western Western Australia on the way through.

Three days of wild weather from Wednesday will include intense winds, rain, thunderstorms and waves with potential for significant damage, according to Weatherzone.

"Wind will be strong enough to damage buildings and bring down trees and power lines,” Dr Dutschke said.

“There is potential for wind gusts to reach 120km/h, rainfall to amount to 60 to 120 millimetres and wave heights to exceed five metres," he said. "The effects will be similar to that of a cyclone but the rain will be less.”

Because the storm doesn't originate in the tropics - cold air from the south is about to interact with warm, moist air from the north – it won't meet the standard definition of a cyclone.

Still, the 100 to 110 km/h winds would give the storm the equivalent of a category-one cyclone.

Towns south of Perth, such as Busselton, may see wind gusts exceeding the 135km/h peak – or category-two cyclonic strength – recorded in the region during big storms there last June, Dr Dutschke said.

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