Best New Year's Eve yet: Sydney mayor

Sydney had its best and most crowded New Year's Eve to date, with more than two million packing into the city for the fireworks display.

An estimated 1.6 million people had been expected to descend on the city for the $6.8 million firework display, but Sydney Lord Mayor Clover Moore said preliminary figures show crowd numbers exceeded expectations.

"We know that the event has just become more, and more, and more popular, and that is something that we will have to take into consideration as we plan for next year," she told reporters on Wednesday.

Ms Moore said her highlight of the night was the midnight show where synchronised explosions saw 1000 fireworks shot from the Opera House sails, as well as glittering waterfalls of fire cascading from the Harbour Bridge.

Thanking the night's creative ambassador Reg Mombassa, Ms Moore said: "I think we can say it was the best ever yet".

But while the majority of partygoers had a safe and happy night, she condemned the assault on an 18-year-old man in Kings Cross.

As the first fireworks display showered over Sydney Harbour at 9pm (AEDT), police say the young man was knocked to the ground in an unprovoked attack.

He remains in a critical condition at St Vincent's Hospital, with his alleged 25-year-old attacker due to face court on Wednesday.

The attack occurred on the same street where teenage king hit victim Thomas Kelly was killed in 2012.

"I absolutely condemn that brutal assault. That assault is close to our consciousness," she said.

Meanwhile, more than 60 trucks, sweepers and 350 staff rolled into the city's CBD to clear away the debris left from the night's celebrations.

"I'm very pleased to say we collected 10 tonnes less (rubbish) this year ... So people are more conscious about that."

With planning already under way for the 2014 show, Ms Moore said this year's event would be reviewed for any improvements that could be put in place.

But she dismissed charging people.

"We see this as the most significant free community event in Australia and it is out goal to keep it that way."