The rate of new coronavirus infections in New South Wales is trending down, but community transmission is on the rise.

In the past week, there has been a 350 per cent surge in infections caught locally with no known link to a confirmed case.

"It's probably the most concerning component of the figure we've had to date," Australia's Deputy Chief Medical Officer Paul Kelly said.

According to Professor Kelly, it justifies the Federal Government's tough new ban on gatherings of more than two people.

As of today, police in NSW have the power to issue on-the-spot fines of $1,000 to anyone congregating in groups of three or more.

It's an extraordinary measure but one other states are adopting too.

If new cases are slowing, is the crackdown necessary?

Here are the numbers health authorities argue justify the crackdown.



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While the data isn't absolute, it provides clues about how the virus is spreading in the community.

Health authorities have attributed the slowdown in new daily cases to a fall in returning travellers and social restrictions starting to bite.

But community transmission cases don't appear to be dropping in the same way.

Tim Newsome from the University of Sydney says infections transmitted within the community have a higher chance of exploding in size.

"Returning travellers are like a drip-feed of new cases, but when the virus becomes established in the community to some extent the cat is out of the bag," Dr Newsome said.

He argues cases that can be traced back to a known contact or an overseas traveller are far easier to control.

It is harder to identify lone cases, especially where people are displaying little or no symptoms.

"If we don't know who has the virus, then each person is infecting around two or three others and that's when you see the real exponential rise," he said.

William Rawlinson from the University of NSW cautions against considering the overall figures in isolation.

"The fact the numbers are dropping is a good sign, but you need to take into account that's the number of positive diagnoses — there might be a much larger number that are untested that have the infection," Professor Rawlinson said.

How low does the number need to go?

Professor Kelly says there would need to be a significant downward trend for restrictions to be lifted, but he stopped short of providing a ballpark figure.

"We want to see the curve not only flatten but bend downwards, and then making that decision about when to take the foot off the brake will be very difficult," he said.

He suggests it could be a staggered approach, but the details have not been finalised.

"How we would make that exit strategy is certainly something we are very much working on at the moment, as much as an escalation strategy," he said.

Either way, he says, current restrictions are likely to last months as Australia watches how other countries manage their next steps.