Most people safely enjoyed spectacular, sanctioned fireworks, but some have started the new year in hospital, and others in custody.
More than a million people swarmed vantage points around Sydney Harbour as nine tonnes of fireworks went up in smoke for one of the world's first celebrations of the new year.
The city's long-time Lord Mayor Clover Moore, while acknowledging her bias, said the team behind the display had outdone themselves.
"I tend to say this every year but it well and truly was our best New Year's Eve ever," she said.
Skies exploded with colour across Australia and revelry continued into the early hours, with hundreds of thousands of Melbourne partygoers also treated to a fireworks and laser spectacular.
Police in the nation's two largest cities noted most partygoers enjoyed the night safely, but officers were kept busy with some violence away from the main celebrations.
A 17-year-old boy was stabbed at Guildford in Sydney's west, where unsanctioned fireworks were let off in a park around 10.40pm.
The boy was taken to hospital in a serious but stable condition, with a search underway for the alleged attacker.
There were some 36 arrests in Sydney's city-centre and harbour foreshore including for assault, affray, robbery and knife possession, and 13 others arrested throughout the police operation.
NSW Police Commissioner Karen Webb said crowds were reasonably well-behaved.
"People were able to come into the city, enjoy their night, and get home safely," she told reporters on Wednesday.
"But a small number of individuals had a bad night, and they are now before the court," she added.
An off-duty senior constable was also charged with mid-range drink driving following a random breath test at Nowra in the state's south.
Victoria Police were pleased with crowd behaviour but disappointed by fireworks-related incidents, including some sparking fires.
There were some 287 reports of fireworks-related incidents across the state, as well as more than 50 assaults.
A teenager is in hospital with non-life threatening injuries after being stabbed on the Mornington Peninsula just before 6pm.
Police said a group fled but two teens were quickly arrested over the Blairgowrie attack, with one charged and another released pending further inquiries.
Police will remain visible across the state on New Year's Day.
Victorian Police Assistant Commissioner Chris Gilbert said most people acted responsibly.
"People really enjoyed themselves, they were generally safe, and they looked after each other out there as well," he said.
Operations also targeted weapon possession, with 14 arrests and 18 seizures including "a couple of swords and a machete" despite police giving advance warning of searches in designated areas.
"There is absolutely no reason for some of those things to be brought into the city ... especially on New Year's Eve," Mr Gilbert said.
Revellers in Queensland and Tasmania were also generally well-behaved, police said.
Some of those who spent the night in hospital still had reasons to celebrate.
Sydney's first baby to be born in 2025 was delivered at 12.32am at the Royal Hospital for Women, with NSW health authorities saying both the mother and baby boy were doing well.
Some 40 babies were born in Victorian hospitals in the early hours of New Year's Day.
In his new year message, Prime Minister Anthony Albanese said the nation could head into 2025 optimistically with wages rising and inflation falling, despite times remaining tough.
Federal Opposition Leader Peter Dutton urged Australians to "reignite our national confidence and our sense of hope" - by electing the Liberal-National coalition.
