Law and order has always been seen as a vote winner in state and territory elections.

Key points: NSW Police Association says more officers are needed to avoid an "epidemic" of violence

NSW Police Association says more officers are needed to avoid an "epidemic" of violence The union warns its officers are at a "breaking point"

The union warns its officers are at a "breaking point" The Bureau of Crime Statistics and Research says there's no evidence to support the union's claim of an imminent crime wave

But how much evidence is there that more police will drop the crime rate?

It is still 10 months until New South Wales goes to the polls, but Premier Gladys Berejiklian made a head start this week, announcing an extra $220 million for the police force, and 100 new positions.

The commitment is well short of the 2,500 extra officers the Police Association of NSW (PANSW) insists is needed to avoid an "epidemic" of violence.

The announcement comes on the heels of a long downward trend in violent and property crime.

The homicide rate is down by 68 per cent, non-domestic assault is down by 35 per cent, and armed robbery is down by 87 per cent on 2,000 levels.

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The national crime rate has also dropped in the same period, albeit by less dramatic margins.

The homicide rate decreased by 37.5 per cent between 2000 and 2016, while property crime dropped by more than 60 per cent in several categories.

PANSW does not dispute the pattern, but warns its members are at a "breaking point", and a failure to increase the number of officers will lead to a disastrous reversal of the current trend.

"We are at that cliff edge where we need to actually invest in policing," PANSW president Scott Weber said.

He argues the state's population growth has outstripped the growth of its police force, 14 per cent to 8 per cent.

"We have the lowest police to population ratio in the country, and also the lowest dollar spent per capita," Mr Weber said.

"The last thing we want to do is have this conversation three or four years down the track, like the Victorians, where their crime rate rose over five years by 25 per cent."

Limited evidence of union's claims, Bureau of statistics says

But the state's crime statistics agency, the Bureau of Crime Statistics and Research warns there's no evidence to support the police union's claim there's an imminent crime wave.

In fact, its executive director Don Weatherburn says there is little data on how police numbers affect crime trends in general.

"There has not actually been too many really good studies on the relationship between the number of police and crime … and the ones that have been done produce wildly divergent results," he said.

"What people have tended to do is wait for a police strike and see what happens. That's maybe 15,000 people stopping work, so naturally you do see an effect.

"But whether adding another 1,000 police … will have a beneficial effect is not a settled issue."

A 2016 Productivity Commission report puts police numbers at more than 70,000, with NSW representing the largest share, at more than 19,000 officers.

That is around 200 officers per 100,000 people.

The same report puts the nationwide cost at $11 billion, or $459 per person.

'It's possible to have too many'

The president of the Australian Council for Civil Liberties, Terry O'Gorman, says it's possible to have too many police.

"The reality is, the police unions are responsible for the never ending law and order campaign," he said.

He argues crime over the last decade has been going down, at the same time as rate of imprisonment has increased.

"If you pump another 30 police into a major police station in Sydney, their superiors will say 'go out on this shift, and I want to see the arrests you've made when you come back'," he said.

"If you increase the traffic policing command … then there's going to be pressure on them to come back with a certain number of tickets per shift.

"It's just the way policing works."