King TIde: Tempe feels the effect of weather pattern.

King Tide: The tide causes mild flooding Holbeach Avenue in Tempe. Photo: Nick Moir

Streets in Tempe and Haberfield were flooded and water splashed onto the wharves around Circular Quay as king tides hit much of Australia's coastline on Thursday morning.

The king tide, an informal name for an especially high tide that happens naturally and predictably a couple of times a year, reached its peak between 9am and 10am.

They are not caused by climate change but they can provide an image of what coastlines could look like in the future, the CSIRO says. The Intergovernmental Panel on Climate Change projects sea levels to rise by between 18 and 79 centimetres this century.

While the ocean lapped at the coast, Australia sweltered through its first heatwave of the year on Thursday. In Sydney temperatures reached 36.5 degrees in the city, the hottest day since last October, while Tibooburra in rural NSW broke a century-old record for the town with a high of 48.4 degrees.

Other states were not spared from the heat. Birdsville in Queensland reached 48.6 degrees and Moomba in South Australia fell just 0.3 degrees short of its record, with a scorching top of 49.3 degrees.

And the community of Longreach in Queensland's south-west sweltered through its sixth day of above 43 degree temperatures, with 44 degrees expected on Friday. That run is expected to continue until Monday, finishing a marathon 10-day stretch of hot weather and a "comfortable" record for the town, Weatherzone meteorologist Ben McBurney said.

Low-lying parts of Wollongong, Newcastle and Lake Macquarie also got a taste of predicted sea level rises during Thursday morning's king tides.

In Lake Macquarie, streets around Swansea Channel were inundated as the tide moved in just after 9am.

Resident Ken Hoff said he had seen the water reach higher than Thursday morning's high water mark in Black Ned's Bay. "It's probably about three inches from where it's reached in the past," he said.

Temperatures in Sydney are expected to cool on Friday in time for the first day of the fifth and final Ashes Test at the SCG, with tops of 27 in the city and 32 in the west.