Wastewater testing in Invercargill and around New Zealand earlier this week has come back negative for Covid-19.

Director-General of Health Dr Ashley Bloomfield, at the 1pm press conference today, there has so far only been positive wastewater results at three Auckland sites.

There were 11 new cases of the Delta variant announced today, bringing the total to 21.

But all are in the Auckland area, and a genome link to the border has now been identified.

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Prime Minister Jacinda Ardern said Ministers will meet in the morning to discuss whether the three-day Alert Level 4 lockdown of New Zealand will need to be extended.

Their decision will be announced at tomorrow's 1pm press conference.

Bloomfield said wastewater testing returned positive results from three sites: Rosedale on Auckland's North Shore and the Western and Eastern interceptors. There are identified cases in all those areas.

"Given we've only got 21 cases so far, the sensitivity of the testing is very high and it does help us with our assessment of the situation.

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"Other sampling undertaken on Monday and Tuesday at Mt Manganui, Tauranga, Hamilton, Rotorua, Gisborne, Taupo, New Plymouth, Napier, Porirua, Hutt Valley, Wellington, Nelson, Christchurch, Queenstown and Invercargill have all come back negative."

Arden said the 48-hour travel window has been extended for both Queenstown Airport and the Cook Strait Ferry for 24 hours, through to 23.59 on Friday, 20 August.

"This applies only to people needing to return home and is in response to the large amount of demand that we have on those facilities," Ardern said.