As Australian health officials scramble to contain the Covid-19 outbreak in Sydney, a top epidemiologist here is warning "that could be us".

Professor Michael Baker says New Zealand is arguably entering "our most dangerous stage" since the August Auckland outbreak as the pandemic surges in the Northern Hemisphere.

Baker, from the University of Otago's school of public health, is now calling for returnees from countries where the virus is "out of control" to take an additional step and isolate under supervision at a hotel and be tested before even stepping on the plane.

The new strain of Covid-19 spreading rapidly across the United Kingdom, forcing a third of its population into lockdown, was further evidence the Government needs to implement a "traffic light" system, he said.

"For New Zealand to get through the next few months until a vaccine is widely available, it has to have another control measure in the source country to try to really reduce the number of infected people arriving here, which is potentially our greatest vulnerability," Baker said, "particularly now there seems the potential for the more infectious virus becoming dominant."

Professor Michael Baker wants more stringent measures for returnees from where Covid-19 is "out of control". Photo / Supplied

The Health Ministry yesterday reported six new cases since Friday in managed isolation facilities among people who've returned from South Africa, Australia, the United States and the Netherlands.

The Ministry said it was also "closely monitoring" the outbreak in Sydney, which reported 30 new cases reported overnight, forcing the cluster's epicentre, the Northern Beaches, into lockdown and the greater Sydney area given restrictions.

Health officials are searching for the source of the northern Sydney beaches outbreak. Photo / AP

New South Wales chief health officer Kerry Chant said finding the source of the northern beaches outbreak might be a "challenge beyond us" despite the extensive investigation.

Officials said they expected numbers to rise.

The New Zealand Government is taking a wait-and-see approach towards the outbreak and what impact it would have on the proposed transtasman travel bubble.

A spokesman said the arrangement wouldn't start until the first quarter of next year but that would depend on there being no significant change in the circumstances in either country.

"We're monitoring the situation closely, but it's too early to make any decisions based on the current community cases in New South Wales."

Baker said arrivals from New South Wales didn't need to be treated differently as proportionately their case numbers were still "minute" and Australia was committed to stamping out the outbreak to continue its elimination goal.

"The main thing is this should be a huge wake-up call for us is that could be us today or tomorrow. We need that continued caution here," Baker said.

"It's quite a throwaway line but while we're on holiday, the virus isn't on holiday. The virus is behaving the way it's always behaved."

As the pandemic surges around the world - especially in the Northern Hemisphere - with almost 21.2 million active cases, Baker said this could arguably be the most dangerous stage for New Zealand since the August Auckland outbreak.

Every infected returnee posed us a threat, he said.

"There's just always the potential for error."

Baker said he wanted the Government to implement a traffic light approach to arrivals so those coming from the highest risk countries had more stringent quarantine measures.

Ideally, he would like those deemed high risk to isolate for about three days in a hotel under supervision and return a negative test before they even got on the plane to come to New Zealand.

The new strain of Covid-19 which forced UK Prime Minister Boris Johnson to cancel Christmas for more than 16 million Brits living in London and south-east England was likely to spread across the UK and Europe, he said.

While it didn't appear to be more deadly or that vaccines and treatments weren't affected by it, scientists believed it was more infectious.

Baker said this "shouldn't come as a huge surprise" because was normal for viruses to mutate but it might make it harder to contain.

"And that is a problem."

Baker said New Zealanders needed to remain vigilant over summer and the most important things to do are:

• if you have cold or flu symptoms, cancel your plans stay home and seek advice about getting a test
• wash your hands regularly
• download and use the Covid Tracer app religiously and turn on the Bluetooth function
• get used to carrying a mask with you and be prepared to use it.

There should be no relaxing in any way of our precautions.