The storm will compound misery in Queensland, which has already been hit by months of flooding that killed 35 people and inundated hundreds of communities. Yasi is due to hit north of the main waterlogged area, but emergency services are already stretched and the whole state is flood-weary.
"This is a cyclone of savagery and intensity," Prime Minister Julia Gillard said in a nationally televised news conference.
"People are facing some really dreadful hours in front of them." Still, many in the storm's path were stoic. Cairns resident Jane Alcorn banned those who planned to shelter with her in the garage of her apartment complex from panicking.
"There's no crying, no hysterics," said Alcorn, 42. "It's going to be loud, it's going to be scary. But we've got each other."
The first of Cyclone Yasi's winds began howling throughout Cairns as night fell Wednesday, with the storm expected to make landfall sometime around midnight.
In Innisfail, a town about 55 miles (90 kilometers) south of Cairns that is nearly in the direct path of the storm, Mayor Bill Shannon said he saw the roof torn off a building near the local government building where some 500 people are sheltering.
"We're just hoping and praying we can all get through the night," Shannon said.
Winds at the center of the storm were gusting up to 186 mph (300 kph), and the front was about 300 miles (500 kilometers) across.
The worst winds were expected to last up to four hours, though blustery conditions and heavy rain could last for 24 hours.
The storm will lash the coast with up to 28 inches (700 millimeters) of rain and send tidal surges that are likely to flood coastal regions, the Bureau of Meteorology said.
The bureau said most at risk was a band about 150 miles (240 kilometers) long between the tourist city of Cairns and the sugar cane-growing town of Ingham. The storm was forecast to continue inland at cyclone strength for two days. It was unclear what the damage to the Great Barrier Reef would be, experts said.
Queensland officials warned people for days to stock up on bottled water and food, and to board or tape up their windows. People in low-lying or poorly protected areas were told to move in with family or friends on safer ground or move to evacuation centers.
"It's such a big storm -- it's a monster, killer storm," Queensland Premier Anna Bligh said, adding that the only previous storm measured in the state at such strength was in 1918. "This impact is likely to be more life threatening than any experienced during recent generations."
Greg McNaught, owner of a used car dealership, hurriedly moved computers and other electronics from the ground floor of his business to his apartment upstairs. His friend Chris Hincksman plastered strips of tape over the large storefront windows.
"We're just going to stay here and drink beer," McNaught said, taking a swig from a can. He admitted being a bit worried by the intensity of Yasi, but held up his drink and said wryly: "Maybe if I've had enough of this, it won't matter."
More than 10,000 people were sheltering in 20 evacuation centers, including one set up in a shopping mall in downtown Cairns, a city with a population of some 165,000. People huddled in hallways with blankets, camping chairs and snacks.
Earlier Wednesday, police told people to get off the streets of Cairns. "Everyone's gotta go now," one officer told pedestrians strolling near the waterfront. "The water is coming NOW."
Warnings stretched as far away as Townsville, which is slightly larger than Cairns and about 190 miles (300 kilometers) to the south, and Mount Isa, some 500 miles (800 kilometers) inland.
State disaster coordinator Ian Stewart said people should move to rooms at the center of their houses -- usually the bathroom -- as they were structurally safest and usually had no windows that could shatter. People should bring mattresses and other items to hide behind in case of flying debris, sturdy shoes, and raincoats in case roofs are ripped off.
Carla Jenkins, a 23-year-old Cairns resident, threw her belongings into a suitcase, taped up the windows of her house and fled to her grandmother's sturdier apartment complex with her sister and her dog, Elmo. The women had a stash of candles, flashlights, water and tinned food, and planned to spend the night huddled in the bathroom away from the windows.
"I can't see many Cairns people sleeping tonight," she said. "Tonight's going to be a very scary night."
The timing of Yasi's expected landfall, just after high tide, meant high storm surges of at least 6.5 feet (two meters) were likely to flood significant areas along the coast, the weather bureau said.
Australia's huge, sparsely populated tropical north is battered each year by about six cyclones -- called typhoons throughout much of Asia and hurricanes in the Western hemisphere. Building codes have been strengthened since Cyclone Tracy devastated the city of Darwin in 1974, killing 71 people, in one of Australia's worst natural disasters.