New Zealand recorded 21 new cases of Covid-19 on Sunday as the outbreak of the Delta variant continued to grow, bringing associated infections to 72, health officials said.

Of the 21 new cases, 20 were in Auckland, the largest city, and one in the capital, Wellington. Five people were in hospital but no one was in intensive care.

New Zealand is under a strict lockdown until midnight on Tuesday as the outbreak has widened beyond the two key cities. The prime minister, Jacinda Ardern, would convene her cabinet on Monday to confirm lockdown extensions for Auckland and Wellington, and consider restriction levels for the rest of the country, AAP reported.

On Sunday the Covid-19 minister Chris Hipkins said scanning in at places and gatherings would become compulsory to aid with contact tracing. Ardern’s government had pushed back against the measure for months.

“We’ve been exploring how best to do this,” Hipkins said, saying the delay was down to overcoming legal, privacy and technical hurdles.

The government will also allow for childcare for households where both parents are essential workers – allowing emergency service workers and border workers to return to the frontline.

The director general of health, Ashley Bloomfield, revealed a potential super-spreader event had taken place at the Samoa Assembly of God’s church service last Sunday. He urged attendees to isolate and get tested urgently.

“Several of the new cases are linked to [that] church service,” he said.

“It’s one of a number of large gatherings and we’ve got several large schools also involved. This is a priority for our contact tracing efforts.”

Bloomfield commended New Zealand’s Pacific community – which has been the centre of previous outbreaks – for responding to the call to get tested.

“They’re very good at mobilising to get tested … by far the highest rates of testing is among our Pacific community,” he said.

Hipkins said about a million people gad been fully vaccinated in New Zealand, after more than 50,000 doses of the vaccine were given on Saturday. “We continue to deliver incredible numbers we can be proud of,” he said.

Until the current outbreak, however, New Zealand’s vaccination pace was the slowest among the OECD, with only a fifth of the population fully vaccinated. The country has recorded 2,660 confirmed coronavirus cases since the start of the pandemic and 26 related deaths, according to the health ministry.

The opposition has accused the government of having left New Zealand a “sitting duck” for the Delta variant by failing to make good use of its six months without Covid-19. “It’s pretty clear that we haven’t used the last year since the August 2020 lockdown to plan for the future,” said Chris Bishop, the National party’s Covid-19 spokesman.

“It took many months to do [quarantine hotel] and airport workers. We still have 35 per cent of frontline port workers who are completely unvaccinated. The fact we’ve got 3,000 healthcare workers in Auckland who are unvaccinated is deeply troubling.”