A monsoonal low off Queensland will drench the state's entire southeast whether it strengthens into a tropical cyclone or not.
Coastal areas south of Rockhampton can expect heavy rain from Thursday night, with up to 300mm - more than the monthly average - expected to hit some areas on Friday.
Strong winds, higher than normal tides and swells up to five metres are also forecast, and residents are being warned to prepare for potential flooding.
The low system was hundreds of kilometres off the Queensland coast on Tuesday and heading in a south-southeast direction.
Bureau of Meteorology forecaster Jess Carey says there's a 20 to 50 per cent chance the low will form into a category one cyclone later this week.
"But these things can change hour to hour," he told AAP on Tuesday.
"Whether it's a tropical cyclone or whether it's a low is not overly important, but what is important is the rainfall that's going to be generated from the system.
"The effects are going to be quite significant."
Areas between Rockhampton and Hervey Bay will be hit with heavy rain on Thursday night before the wet weather heads south.
Coastal areas south of Bundaberg down to the NSW border and inland to the Great Dividing Range can expect 100-200mm on Friday, with up to 300mm in some areas.
"There's the potential some areas will get their monthly average rainfall in a day," Mr Carey said.
"There could possibly be flooding and then on the coast you'll have beach erosion, beach closures, all these types of issues.
"Friday is the real focus but potentially by Saturday morning it could be relatively dry."
Most northern areas won't be affected, although heavy rain is forecast for parts of Cape York over the coming days as monsoonal trough moves in.
Cyclone Lam, developing in the northern Gulf of Carpentaria, is expected to move away from Queensland and deliver rain primarily to the interior of the Northern Territory and Western Australia.
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