The threat of a rare cyclone developing off the southeast Queensland coast has emerged as authorities step up the search for a "good Samaritan" feared missing in raging floodwaters.
The Bureau of Meteorology estimates there's a 50 per chance a low pressure system over Double Island Point, south of Maryborough, will develop into a cyclone off Fraser Island.
Severe weather manager Vikash Prasad says it's only a "weak" chance, although the situation will become more clear over the next 24 to 48 hours.
"It's going to move offshore in the next 24 hours and there's the weak possibility it may develop into a cyclone," he told AAP on Monday.
"But it's very, very hard to see a system developing into a cyclone so far south." Mr Prasad said even if the cyclone forms to the east of Fraser Island, it will be moving east and north, away from the coast.
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"We will see the conditions on the Sunshine Coast and the Brisbane area will continue to ease," he said. If the cyclone does develop, Fraser Island would bear the brunt of the heavy weather.
A severe weather warning is in place for the island and the southeast Queensland coast. Meanwhile, authorities are searching for a "good Samaritan" reportedly missing from where a man was swept to his death north of Gympie.
The Department of Community Safety (DCS) said bystanders spotted another person at Glenwood, between Gympie and Maryborough, where the man died in floodwaters on Monday morning.
"They've said there was a good Samaritan there on the scene when the four-wheel drive went over the causeway, but we've been unable to confirm those reports," a DCS spokeswoman told AAP.
Authorities have called off the search for a third person after it transpired another car had been parked and unoccupied when it was swept away in the deluge. The area around Gympie copped heavy rainfall on Sunday and Monday.
Goomboorian, north of Gympie, has recorded 340mm of rain since Sunday morning. Gympie Mayor Ron Dyne said flooding at Tinana Creek, north of Gympie, is expected to exceed the flood event of 1992.
The bureau said the deluge was likely to continue for the next 24 hours. Forecaster Tony Auden said there was a low pressure system on the coast north of Gympie.
"We expect heavy rainfalls to the south of that low which covers much of the Sunshine Coast and the Gympie area," he told AAP.
The Bruce Highway has been cut at several points north and south of Gympie, among dozens of road closures in the area.
