Queensland's swollen Mary River is expected to peak at more than 17 metres but lower than the mark set during the state's last major flood.
The Bureau of Meteorology says floodwaters aren't expected to reach the 2013 mark of 19.98 metres.
Forecaster Jess Carey said some parts of the town of Gympie and outlying properties would likely be affected but not the CBD.
The floodwaters are also expected to cut the Bruce Highway, dividing the town in two.
"Gympie is one of the most flood-prone towns in Queensland and they're pretty good at mopping up after floods because they have to do it so often," Mr Carey told ABC Radio on Saturday.
"They've really had quite a trying time over the last few years, that's for sure."
Fraser Coast Mayor Gerard O'Connell was preparing for moderate flooding downstream at Maryborough on Sunday night.
"We're looking at something between a five and eight metre flood," he told AAP.
Mr O'Connell said any flooding of about 5.5 metres would disrupt the town.
He said the Lamington Bridge linking the CBD with the western suburbs could go under and the suburb of Granville could be isolated.
The mayor said he would have a clearer picture of what to expect after a disaster committee meeting on Sunday morning.
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