The torrential rains eased on Thursday and floodwater receded, but authorities and locals are faced with a mountainous clean-up effort.
Relentless rains since late last month burst river banks across Australia's south-east, submerging homes, farms and bridges, and cutting off entire towns. At least 21 people have been killed so far.
"Australia is becoming a harder country to live in because of these natural disasters," Prime Minister Scott Morrison said on Wednesday.
The emergency declaration, which was set up after destructive bushfires in 2019, will help to cut red tape and speed up aid after criticism about a slow response to the floods.
Frustrated residents in the Northern Rivers, with no access to power and internet for several days, blame authorities for the slow speed and scale of relief efforts.
Mr Morrison, who is trailing in polls ahead of a federal election before May, kept media away from his meetings with flood victims, which he said was to protect their privacy.
Television footage showed some people gathered in front of an emergency operations centre Mr Morrison visited, yelling "the water is rising, no more compromising" and "fossil fuel floods".
Mr Morrison's conservative government late last year adopted a net zero carbon emissions target by 2050, but climate activists are demanding more aggressive action.
Mr Morrison linked the devastation to climate change, which he said had also caused previous bushfire catastrophes, but he went on to say the greater challenge was reducing other countries' emissions.
What will save people is flood mitigation works, rather than tougher curbs on Australia's emissions, he said.
Officials said military personnel sent to the region to assist the clean-up operations would be more than doubled to 4,000.
The government has paid out about A$385 million ($282.9 m) to flood victims nationally in the past week, and Mr Morrison said aid would be increased in Lismore, one of the hardest-hit towns, and surrounding areas, to provide food and shelter, mental health support and legal and business support.
"We know it has been a devastating time up here, probably moving through the initial shock for many people and the trauma that is associated with that," New South Wales state Premier Dominic Perrottet said from the worst-hit Northern Rivers area.
"Many people are now coming back to their homes in very difficult conditions, many are not habitable," Mr Perrottet said as he unveiled an aid package providing up to 16 weeks of rental support.
More than 1,200 people are in emergency accommodation in the Northern Rivers region while about 3,000 homes were deemed uninhabitable, authorities said.
Rescue teams, including defence force personnel, took advantage of better conditions to clear debris and deliver essential supplies, but many residents, with no access to power and internet for several days, were angry.
A Climate Council of Australia report published on Thursday described the recent flood events as one of the most extreme disasters in Australian history and said the devastation was "wide ranging".
Total damages are estimated at A$1.77 billion, the Insurance Council of Australia said.