New Zealand has conceded its ambitious “Covid zero” 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.

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.