Early attendance figures suggest the number of people who came to the city to watch the New Year's Eve display might have been half that of last year.

Sydney lord mayor Clover Moore would only say "over a million" people came into the city on Wednesday night.

That appears to be significantly lower than the city's last-minute projection of 1.6 million people, and the attendance figure might have been half last year's estimated record-breaking crowd of 2 million.

Jon Corbishley, a consultant on crowd management, said the city's management of the event had been largely unchanged.

"I think there are some external factors at work here," he said. "It's possibly the threat of terrorism. The Martin Place [siege] was a first for Australia.  It may have had the effect of making people stay at home."

The Prime Minister, Tony Abbott, issued a terrorism warning on New Year's Eve, a little more than a week after three people were killed following a 17-hour siege in the centre of Sydney's financial district.

There were also reports on New Year's Eve that authorities had detected increased terrorism "chatter".

But the state's Premier, Mike Baird, denied the turnout was influenced by any sense of fear.

"People make their own plans according to their own circumstances," he told reporters on Thursday. "Whether numbers were slightly up or down, I don't really think that's an indicator [of fear]."

Authorities in Melbourne and Brisbane also reported smaller crowds at New Year's Eve events.

The lord mayor said Sydney's crowd estimates were provisional. But unlike previous years, the city declined to provide updated estimates or further comment on Thursday.

Cr Moore nonetheless praised Sydney's New Year's Eve as a success.

"The fireworks were impossibly better than ever," she said on Tuesday. "Sydneysiders have gone through a period of trauma and mourning over the past weeks, and last night we all spared a thought for the families of the Martin Place siege victims."

The city's annual early morning clean-up also provided indications of diminished crowd numbers.

More than 350 staff and 60 trucks worked through the night to clean the streets, Sydney council said.

They collected a little less than 30 tonnes of rubbish from harbourside parks but only 20 tonnes from around the city, or about 10 tonnes fewer than expected.

Mr Corbishley, who has consulted to governments and companies on event planning, including one involved in organising this year's New Year's Eve festivities, said increased alcohol restrictions may have turned some people away.

Six vantage points were jointly designated as alcohol-free zones a week before this year's celebrations. Police were granted the power to confiscate BYO alcohol from revellers over the holiday period.

But laws "locking out" patrons from the CBD and Kings Cross were relaxed, to allow people entry until 3 am.

Transport in the city's centre was plagued by overcrowding and delays shortly after midnight.

A fire scare stopped some trains for up to 20 minutes while emergency services responded to an alarm.

A number of stations in the city's east and centre were shut for 20 minutes after being overwhelmed by commuters.

Prices for UberX, the controversial new "ride-sharing" service, surged to nearly four times normal fares.

A trip of about 25 kilometres from the Sydney Opera House was going for a fee of $90 about 1am.

with Pallavi Singhal