The city of Rockhampton in central Queensland is almost completely isolated by floodwaters.
The Fitzroy River is expected to peak at 9.4 metres on Wednesday, a level similar to floods that occurred in 1991 and 1954.
Hundreds of homes are already inundated and people have been forced to leave.
The ABC reports the Bruce Highway to the north of the city is still open to traffic. Police say it might stay open until Tuesday evening.
Rockhampton is one of 22 cities or towns in Queensland that have been inundated in the crisis.
The BBC reports approximately 850,000 sq km have been affected, an area equivalent in size to France and Germany.
The ABC reports the floods have claimed the lives of 10 people since they began.
State Premier Anna Bligh told the 7.30 Report on ABC TV on Monday that the last route out of Rockhampton - the highway to the north of the city - was under water, completely stranding the city's 75,000 residents.
The airport was closed days ago after the tarmac became flooded and no rail services are operating.
"We're really still in the middle of an unfolding disaster," she said.
Ms Bligh said the clean-up efforts in all the flooded towns are going to be a mammoth task that will continue for a long time.
"There's certainly some big logistical issues," she said.
"The city of Emerald, for example, we flew 13 tonnes of cleaning equipment and disinfectant in there this morning and they need more. So that's just one town.
"There's a lot of work and it's a big rebuilding programme, and some of them (the residents) ... will not be back in their homes for months.
"Our job is to rebuild regional Queensland and that's what we're going to do," she said.
NZ aid offer
Australian Prime Minister Julia Gillard says the New Zealand and United States governments have offered their assistance to the state.
She says New Zealand prime minister John Key contacted her to offer support and the US embassy in Canberra has also passed on an offer of assistance.
But Ms Gillard says international aid is not necessary yet.
