UPDATE: FLOODWATERS have cut Gympie in south eastern Queensland in two, with about a dozen businesses under water.
Acting Gympie Regional Mayor Tony Perrett says up to 40 businesses and 40 homes could be inundated if the Mary River reaches 20 metres, as expected, in the next 24 hours.
The Bureau of Meteorology has warned the river could go even higher than that.
If it gets to 21.9 metres it will match the 1999 flood that saw homes and businesses inundated and the city declared a natural disaster area.
But Mr Perrett said Gympie had seen far worse floods in the past, including a 25.5-metre peak in 1893.
He told AAP about a dozen businesses just off Mary Street were inundated this morning, and up to 40 in the CBD could follow.
About 40 homes could also be awash with floodwaters if the 20-metre peak eventuates.
Mr Perrett said the community was well prepared, and businesses began moving their stock off the ground when heavy rain began on Friday.
Emergency services personnel have been doorknocking residents in at-risk areas, saying they must be prepared to evacuate.
An evacuation centre has been set up to accommodate residents leaving their homes.
"We are working on the basis that it's going to get to 20 metres," Mr Perrett said.
"Gympie is really quite well versed in handling floods, we activate as the river rises ... the community gets cut in half and we have to position ourselves on both sides of the river to respond."
Among the businesses already flooded is the Royal Hotel on the corner of Mary Street.
During the 1999 flood, it was submerged to its roofline, and assistant manager Jess Philpot expects about the same this time.
"You want to cry," she said today, as about 1.5 metres of water washed through the pub's bottom bar.
"It's going to go up to the roof. In '99, it was lapping at the awnings."
She said she was stunned at how quickly the water had risen.
"Last night we couldn't see the water from the hotel. This morning it's inside," she said. "We haven't slept. We sat up and watched it coming up but we didn't think it would be so quick."
She said there'd been plenty of warning though, allowing stock and other items to be moved out.
The Bruce Highway is cut to the city's north and south, and police are urging people not to attempt to travel to Gympie.
The Mary River catchment saw a massive dumping of .32 of a metre (322mm) in the 24 hours to 5am (AEST) today.