The Fitzroy River rose to 8.75m (29ft) in the central Queensland city on Thursday.
It comes nine days after Cyclone Debbie made landfall in northern Queensland, causing floods along a 1,200km (745 miles) stretch of coast.
Authorities said Rockhampton would face a "major flood" for at least a day.
"This body of water is incredible," Queensland Premier Annastacia Palaszczuk said.
