Auckland has been plunged back into its second hard lockdown this month after a man tested positive to the coronavirus with an unknown origin.

The city’s 2 million residents woke on Sunday to an “unfortunate but necessary” seven-day shutdown only a fortnight after emerging from three days confined to home.

The rest of New Zealand will be under level two restrictions which limit gatherings, among other rules.

One of the first casualties of the lockdown was America’s Cup racing between Team New Zealand and Italy’s Luna Rossa, which organisers said they had no choice but to postpone.

The first two race days, originally scheduled to take place off Auckland next weekend, have been pushed back “to provide at least some certainty in planning for all event stakeholders,” America’s Cup Event (ACE) said in a statement.

Prime Minister Jacinda Ardern said the latest patient visited a gym while displaying symptoms, raising fears that others might have contracted the virus through casual contact.

The 21-year-old man, dubbed ‘Case M’, first developed symptoms on Tuesday but may have been infectious almost a week ago dating to Sunday.

“Based on this, we are in the unfortunate but necessary position to protect Aucklanders again,” Ms Ardern said.

The New Zealand Herald reports the infected man attends the Manukau Institute of Technology (MIT) and went to MIT for three days and to the gym twice, including once after taking a COVID test when he should have been isolating at home.

He is the older brother of a student at Papatoetoe High School which was the focus of several earlier cases, including those that prompted the last lockdown.

That high school student has returned three recent negative tests and is asymptomatic.

Public health officials are now interviewing Case M to try and determine how he became infected and whether it may be linked to the last cluster which infected 12 people.

That cluster was linked to a family of three who tested positive to the highly infectious UK strain, the first time that mutation was detected in NZ.

New Zealand, one of the most successful developed nations in controlling the spread of the pandemic, has had more than 2000 cases of the coronavirus since the start of the pandemic.

A marquee Twenty20 cricket clash in Auckland between New Zealand and Australia, scheduled for Friday, will be played in Wellington without crowds, New Zealand Cricket said.

The new restrictions also complicate the America’s Cup Event yacht race scheduled to start on March 6 in Auckland’s harbour.

America’s Cup Event said on Twitter it was working “through the implications”.

-with AAP