A growing cluster of COVID-19 in Sydney's Northern Beaches is not expected to burst plans for a trans-Tasman travel bubble, according to an expert.

The cluster increased to 41 cases on Saturday, with officials ordering a three-day lockdown for locals. 

From late Saturday afternoon until midnight Wednesday, residents are only permitted to leave their homes for five essential reasons: medical care, exercise, grocery shopping, work, or for compassionate care reasons.

The latest outbreak has put a question mark over the New Zealand Government's plan to open an air bridge with Australia in the first quarter of 2021. However, infectious disease physician Mark Thomas says the March deadline is still far off - and it's too soon to get concerned about possible ramifications on travel.

"I think this doesn't necessarily change the plan for a travel bubble in March - March is a good, long time away and hopefully by the time February's here, this will be a distant memory," he told Newshub on Saturday.

"[Outbreaks] will continue to pop up and they'll no doubt cause concern about the travel bubble - but I think those are complex political decisions more than public health decisions."

However, that's assuming the outbreak is stamped out quickly, he said.

"We don't want to be placed at extra risk and if this outbreak is not quickly brought under control, I think it will have a delaying effect on the travel bubble," Thomas added.

He noted that with the appropriate measures in place, the cluster should be controlled relatively quickly.

"I think we will see more cases. It's growing but it's not necessarily going to grow any faster in the coming days. It sounds as though they've taken very appropriate measures to try and bring it under control quickly and people are responding, by and large, very well to what's been recommended. It is also in a relatively isolated area of Sydney," Thomas said.

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"I think the Australians have learnt from experience, as we have - you keep pushing on with these outbreak controls and you will get there."

Yet Christmas Day could look a little different for the Northern Beaches, Thomas added, and residents should be prepared for the possibility that restrictions will be extended.

"It could impact their Christmas Day," he said. "I think there [still] might be some degree of lockdown in that area of Sydney."

The source of the cluster is currently unknown and while this can be "disconcerting" it's "not the end of the world", Thomas said. The origin of Auckland's outbreak in August still remains unclear, but it didn't impact the ability to tackle the virus effectively.

"We don't know the original source of the August outbreak, but we brought it under control. You don't need to know the source - you just need to keep going by identifying cases, isolating them and quarantining their contacts."

All but two of Sydney's new cases are from the Avalon cluster, named after a community of roughly 10,000 people on the Northern Beaches about 40 kilometres from downtown of the city.

On Saturday, New South Wales state Premier Gladys Berejiklian told reporters that it's hoped the three-day period will give officials time to crack down on the virus, allowing locals to enjoy the holiday festivities.

"We’re hoping that will give us sufficient time to get on top of the virus so that we can then ease up for Christmas and the New Year."