t's likely New Zealand's pandemic alert level 4 lockdown will extend past four weeks, especially if people continue to flout social distancing rules, experts say.

The number of confirmed new COVID-19 cases has trended downwards in recent days, a week after the strict measures were put in place. But it's not clear yet whether that will continue as testing ramps up and people infected before the lockdown - and those they've passed it onto - start to show symptoms.

"The numbers are still pretty small," said University of Auckland professor Shaun Hendy, whose expertise includes analysing complex systems and mathematical models. 

"I know we're looking at the growth of numbers overseas - countries like the United States, countries in Europe like Italy. That's really where the spread is out of control. We're not in that situation at the moment... we've got a degree of control in place, so we're not seeing the same growth as we are in those countries."

Researchers in Australia - where there have been nearly 5000 recorded cases and 21 deaths - have crunched the numbers on social distancing to see if it really can hold back the spread of the virus behind COVID-19, which has killed 46,000 people around the world already.

They found if 90 percent of people follow the guidelines - which include staying at least 2m away from people outside your home 'bubble' and limiting time spent away from home - the spread of the SARS-CoV-2 virus could be under control by May.

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But if only 80 percent comply, it could take until July - with all the extra economic damage that brings.

If fewer than 70 percent comply, the virus will spread uninhibited, the research found. 

"If less than seventy percent of the population is adopting social distancing measures, we cannot suppress the spread of the pandemic and any social distancing could be a fruitless effort," said Professor Mikhail Prokopenko of the University of Sydney, who took part in the research.

"There is a clear trade off - stricter measures imposed earlier would reduce how long our lives are impacted by this disease. On the contrary, laxer protocols could mean a longer, more drawn out and ineffective struggle against COVID-19."

Experts say quarantining and social distancing are literally the only options we have at this stage, without any vaccine or proven treatment yet available.

"These strategies together with reliable testing have enabled South Korea to emerge from lockdown but they are aware of a possible second wave and are testing it," said Robert Webster, virologist at St Jude Children's Research Hospital in the US. 

"Evidence for quarantine is centuries old," said University of Otago pathology lecturer Ayesha Verrall. 

Dr Hendy said although the paper is yet to be peer-reviewed, that's understandable in the current climate. 

With New Zealand's figures still low - 708 confirmed cases, the vast majority linked to overseas travel - Dr Hendry says that "makes it hard for people like me to figure out what's going on".

"I'm very happy to leave it that way. If we eliminate the disease and it goes away and I don't get any more data, that's fine with me."

COVID-19's R0 figure is about 2.5 - that means for every person who gets it, they'll infect another 2.5 people on average. But with social distancing, it's hoped the R0 can be brought below 1, which would eventually see it wiped out. 

Dr Hendy believes there remains a "strong possibility" the lockdown will go past the initial four weeks, but perhaps only in parts of the country where clusters of the disease have been recorded.

The call to extend could come late however - Dr Hendy says it could be a few weeks before evidence emerges on whether Kiwis are doing enough to slow the virus' spread. 

"All the models show that even though it may appear the disease is under control as case numbers drop, it can bounce back after controls are lifted."

The Australian research can be read here.