The flood level is expected to peak on Sunday afternoon.

Hydrologist at the weather bureau's Flood Warning Centre, Sue Oats, said significant flooding is now predicted after rain from ex-Tropical Cyclone Marcia caused the Mary River to swell.

"That, on top of the Friday rainfall, has bumped river levels at Gympie up from what was anticipated to be a moderate flood peak, to now a major flood peak."

Floodwaters cut the Bruce Highway to the north and south of the city earlier on Saturday afternoon.

There were diversions for small vehicles but not for B-doubles, but the diversions were predicted to be cut too when the Mary River peaks later on Saturday evening.