Tropical Cyclone Alfred was 250 kilometres (155 miles) east of busy Brisbane city on Thursday afternoon, but government forecasts warned its slow and "erratic" crawl towards the mainland was growing difficult to predict.
Some four million people were in the firing line along a 400-kilometre (250-mile) stretch of coastline expected to see the worst of the storm.
"We're already seeing gales developing on the coastal fringe," Bureau of Meteorology forecaster Sarah Scully told AFP.
"There have been very large waves and powerful swells. That's generated by Alfred lingering in the Coral Sea and creating a whole lot of wave energy."
A 12.4-metre (40-foot) wave was recorded on the Gold Coast south of Brisbane, the largest swell ever picked up by that monitoring station.
Daring surfers paddled out to catch the supercharged waves, ignoring the threat of US$10,000 fines for "reckless behaviour".
"I am just staggered that people would be so stupid. It is a huge act of stupidity," said acting Gold Coast mayor Donna Gates.
- Wild weather -
Alfred was initially forecast to strike land late on Thursday evening.
But the slow-moving storm -- churning towards the coast at just 7 kilometres per hour (4 mph) -- was now more likely to make landfall late on Friday or early Saturday.
While this gave coastal hamlets more time to stack sandbags and stockpile food, Scully warned it also left them exposed to wild weather on the storm's outer edges.
"It will mean that the coastal areas are exposed for a longer period of time," she said.
Some towns had already seen "well over" 200 millimetres (8 inches) of rain, Scully said.
Alfred would cross the coast somewhere near the bustling metropolitan hub of Brisbane, the weather bureau said.
It would be the first cyclone to make landfall in the area for more than 50 years.
"There's a lot of people in harm's way here. We're talking about something like four-and-a-half million Australians," said senior government minister Jim Chalmers.
"It's rare for a cyclone to be this far south and to threaten such a huge population area."
- Big Prawn rattled -
American rap star Ice Cube was caught in the storm ahead of concert dates in southern cities Sydney and Melbourne.
"I'm staying clear of it. I haven't been through a cyclone before. I can check this off my bucket list for sure," he told Australia's Channel 10.
"I'm right here on the beach. The beach looks like it is going to be in the lobby in a minute."
Strong winds knocked out power to more than 4,000 homes, utility company Essential Energy said.
More than 900 schools across Queensland state and the flood-prone northern rivers region of neighbouring New South Wales were closed on Thursday morning, education department officials said.
And gusts rattled the beloved "Big Prawn" sculpture in the coastal town of Ballina, snapping a fibreglass feeler off the nine-metre (30-foot) tall crustacean installation.
While cyclones are common in the warm tropical waters lapping Australia's northern flank, it is rarer for them to form in cooler waters further south.
Alfred would be the first to make landfall in that part of Australia since 1974, the Bureau of Meteorology said.
Researchers have repeatedly warned that climate change amplifies the risk of natural disasters such as bushfires, floods and cyclones.
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