Thousands of homes could be flooded in Gladstone from water gushing out of a nearby dam and a king tide.
The Awoonga Dam has broken its bank and is spilling water into the Boyne River.
The suburbs of Tannum Sands and Boyne Island, at the mouth of the river, are being evacuated before Saturday morning's 4.2 metre high tide.
Mayor Gail Sellers has called the situation unprecedented.
"The tide will back up there," she told ABC radio.
"At the moment, police and emergency services are down in that area door-knocking. They are asking people to evacuate."
The Local Disaster Management Group at Gladstone said flood levels along the Boyne River will exceed one in 100 flood levels on the morning high tide at 8.45am (AEST) on Saturday and evening high tide at 9pm.
The SES has received about 650 calls for help, with over 400 of those from Gladstone and Rockhampton.
Towns west of Gladstone received about 400mm of rain since 9am on Friday.
Another 400mm of rain fell on Rockhampton over two days and strong winds brought down trees and branches.
It has closed the Bruce Highway in several places between Rockhampton and Gladstone, and phone and internet connections are down in places.
Queensland Rail says the train line is cut near Rockhampton and there are no services between Brisbane and Cairns.