Nearly 700 extra ADF personnel are set to arrive in flood-affected areas of NSW and Queensland as senior ministers meet to finalise further financial support measures.
A further 680 troops will assist with clean-up efforts by the end of Tuesday.
It's expected there will be as many as 5000 ADF personnel deployed across flood-ravaged areas by the end of the week.
It comes as federal cabinet's expenditure review committee will convene later on Tuesday, where flood assistance will be discussed.
Prime Minister Scott Morrison said the government was finalising the first wave of what he described as "over and above measures" - disaster funding that is outside of the existing categories of support.
Mr Morrison said the plans would be integrated alongside similar support from the NSW government.
"The Australian government will stand with those communities in the northern rivers to support them as they clean up now and in the months and years ahead," he said.
"This is not just a flood event ... this is a catastrophe on a national scale."
As of Tuesday morning, more than $282 million in disaster payments to flood victims have been paid to 242,000 people.
Of that, $157 million has been paid to victims in NSW and and $125 million in Queensland.
Opposition Leader Anthony Albanese criticised the government's handling of bringing in the ADF.
"Clearly, there have been issues here with people who were on the roofs of places for a long period of time. There's a need for an explanation," he said.
"Why are people having to pitch in and privately hire helicopters when the ADF have access to those resources? They are all questions that need answers."
Mr Albanese said the government had failed to use money from its emergency response fund.
Defence Minister Peter Dutton defended the timing of the ADF's arrival, arguing many areas were inaccessible.
"It's not just a couple of streets or a couple of towns or anything like that, it is a broad area," Mr Dutton told ABC radio on Tuesday
"Despite the magnificent work (ADF troops) have done, I think they just became overwhelmed with the scale of it."
Mr Dutton said ADF assistance would increase as floodwaters subsided.
"We need to be realistic about the situation in northern NSW. It's actually not just a flood, as others have pointed out - this is more of a cyclone," he said.
"There were roads that are no longer there, houses have been condemned, these are circumstances in which they're dealing and it will take time to respond and rebuild."
ADF troops coming into flood areas will spend the coming days helping to clear roads and fix telecommunication networks and will use helicopters to drop critical supplies to cut-off communities.
