For police, it is a major operation in keeping revellers safe, for doctors and paramedics it is the most hectic time of year, and for some partygoers it is a night that ends in the back of a divvy van or an ambulance.

Extra police and paramedics will be rostered on this Saturday to cope with the surge in demand that comes from hundreds of thousands of people going out to mark the new year.

And while Victoria Police deputy commissioner Andrew Crisp is quick to point out that the overwhelming majority of revellers do the right thing, there are still some who go out and cause trouble.

"New Year's Eve for us is one of the major police operations for the year," he said.

With all the bustling crowds and masses of activity, police also using the opportunity to flip the switch on their latest weapon in the fight against crime.

Victoria Police's high-tech new monitoring system draws in multiple sources of information - such as social media posts, CCTV footage, weather data, live pedestrian counts, search trends and more - which an around-the-clock team of analysts will scour for any signs of trouble.

The $15 million dollar centre, which the Andrews government funded in the wake of the Moomba riots in March, goes live Saturday night in time for New Year's.

"It will be an invaluable resource for us for the New Year's operation… it will allow us to pull together the threads," Deputy Commissioner Crisp said.

He said significantly more uniformed and plainclothes police would be out in force this year, adding there was no specific terrorist threat against New Year's celebrations.

A total of 126 people were arrested over New Year's throughout Victoria last year, 65 of them for being drunk and disorderly. He said there were also arrests for minor assaults, graffiti and other low-level crimes.

But while crime numbers over New Year's were down last year, crime data shows there was a spike in assaults and sexual assaults in the last week of 2013 and 2014. In 2013, there were triple the average number of sexual assaults in the final week of the year.

It is a similar story in Sydney, where New Year's Eve is the worst night of the year for violent offences, according to New South Wales crime statistics bureau head Don Weatherburn.

"The combination of crowds, heat, alcohol and testosterone is toxic," he said. "Add methamphetamine into the mix and you've got a recipe for big trouble."

The 48 hours spanning December 31 and January 1 is also the busiest time of the year for ambulances, as heat and drug-fuelled celebrations combine to put extra strain on paramedics.

Ambulance Victoria state commander Paul Holman said many of these cases were from people who simply didn't take care of themselves.

"Let your hair down, by all means, but take precautions to avoid seeing in the new year in the back of an ambulance."

Hospitals are bracing for more patients as well, and will likely face three times more Victorians than normal showing up in emergency departments with alcohol-related problems.