Officials in New Zealand have said the country's ambitious 'zero Covid’ elimination strategy may no longer be viable, as an outbreak of the virulent Delta variant spread further.

Covid-19 Response Minister Chris Hipkins reported a further 21 cases in a virus cluster that emerged in Auckland last week, ending a six-month run of no local cases and sparking a national lockdown.

Mr Hipkins said Delta's highly transmissible nature was making this outbreak more difficult to contain than others, raising "big questions" about the elimination strategy.

"The scale of infectiousness and the speed at which the virus has spread is something that, despite all the best preparations in the world, has put our system under strain," he told TVNZ.

New Zealand's widely praised Covid-19 response - which has resulted in just 26 deaths in a population of five million - centres on eliminating the virus from the community.

It has relied on strict border controls backed by hard lockdowns when any cases do slip through, but Mr Hipkins said Delta may force a rethink.

New Zealand Prime Minister Jacinda Ardern during an update earlier today

"(Delta's) like nothing we've dealt with before in this pandemic," he said.

"It does change everything, it means that all of our existing preparations begin to look less adequate and raises some pretty big questions about the future of our long-term plans."

Neighbouring Australia has also pursued a ‘zero Covid’ strategy and been similarly frustrated as its Delta cases continue to spike.

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The New Zealand outbreak has underlined the country's slow vaccination roll-out and prompted accusations the government became lax after its early success dealing with the pandemic.

Only about 20% of the population are fully inoculated, one of the lowest rates in the developed world.

Opposition National Party spokesman Chris Bishop said the outbreak had exposed a lack of urgency in Wellington's vaccine plans.

"The government's complacency and inability to ensure supply and delivery of the vaccine has made us all sitting ducks, completely vulnerable to the Delta variant when it inevitably got into the community," he said.

Another opposition figure, ACT Party leader David Seymour, said Mr Hipkins could not use the Delta variant as an excuse for current failures.

"We've known about Delta since December, what's he been doing in the meantime?" he asked.

Mr Hipkins said elimination remained top priority for the ongoing Delta outbreak, which now totals 72 active cases - 66 of them in Auckland and six in Wellington.

Positive cases have been recorded visited several busy locations, including schools, churches and supermarkets, with health teams checking the status of almost 9,000 close contacts.

The national lockdown is due to expire late Tuesday, although Mr Hipkins indicated Auckland could face further restrictions even if they were lifted elsewhere.

"If I was an Aucklander, I'd certainly be preparing to be at home for a bit longer," he said.

A Covid-19 testing site in Christchurch, New Zealand

Meanwhile in Australia, Prime Minister Scott Morrison has defended his country's lockdown strategy, saying it would stay until at least 70% of population is fully vaccinated, as daily infections break records.

Today’s 914 cases of the highly infectious Delta variant surpassed the previous high of 894 a day earlier.

"You can't live with lockdowns forever and at some point, you need to make that gear change and that is done at 70%," Mr Morrison said in a television interview on the Australian Broadcasting Corp.

Lockdowns are a key element of the federal government's strategy to rein in outbreaks until the 70% percent level is reached, with borders being reopened gradually when the figure climbs to 80%.

But they are taxing the patience of many.

Police arrested hundreds of people yesterday during anti-lockdown demonstrations in Sydney and Melbourne, the capitals of the two most populous states, New South Wales and Victoria, which are under a strict lockdown.

Victoria, in its sixth lockdown since the start of the pandemic, has recorded 65 locally acquired cases, taking the tally in its current outbreak to 440 active cases.

"We are throwing everything at this," said Martin Foley, the health minister of the southeastern state.

New South Wales saw 830 new infections today, despite stepped-up efforts, and the Australian Capital Territory, home to the capital, Canberra, had 19. Nationwide, the tally of active cases stands at nearly 12,000.

Just about 30% of Australians older than 16 have been fully vaccinated, health ministry data showed yesterday. This is mainly because the Pfizer vaccine is in short supply and the AstraZeneca vaccine provokes public unease.

The pace has picked up recently, as supplies increase and Delta spreads. A Newspoll this month for The Australian newspaper showed that 11% of respondents would flatly refuse vaccination.

In New South Wales, at least 57% of those eligible have received one dose, while 30% are fully vaccinated.

"If our community keeps getting their vaccines the way they are, New South Wales will look pretty good by October, November," said state Health Minister Brad Hazzard.

Despite a third wave of infections from the Delta variant, Australia's Covid-19 numbers are relatively low, with just under 44,000 cases and 981 deaths.