New Zealand’s COVID-19 outbreak worsened to 21 cases on Thursday, but Prime Minister Jacinda Ardern claimed a significant breakthrough in determining the source of the crisis.

Director general of health Ashley Bloomfield’s predictions of another jump in cases were realised at the daily briefing, when he announced a further 11 cases.

New Zealand’ first community case in 170 days – and first of Delta – was identified on Tuesday, prompting Ms Ardern to begin a nationwide lockdown.

Little information has been provided of the 11 new cases, though Dr Bloomfield confirmed five were a family with a link to Avondale College, where a teacher with COVID-19 works.

All 21 cases are Auckland based, and a majority are aged under 30.

The young people have socialised heavily, with more than 100 potential exposure sites listed as locations of interest, including cafes, bars, the casino, cinemas and restaurants in Auckland and the Coromandel.

“Because of the demographic of the cases, they have been out and about, there’s real potential for widespread [transmission],” Dr Bloomfield said.

Some 326 people have been categorised as close contacts, a number that would rise to about 1000 by the end of Thursday, he said.

However, Ms Ardern revealed some good news: Headway on deciphering the likely origin of the outbreak.

Genomic sequencing has revealed a strong link to a man who returned to New Zealand on a “red zone” flight from Sydney on August 8, and tested positive at during his fortnight in hotel quarantine on arrival.

“Our cases are most likely to have come from the traveller,” Ms Ardern

Officials are focusing contact tracing efforts towards the quarantine hotel, Auckland’s Crowne Plaza, and its facility for confirmed cases, Auckland’s Jet Park hotel.

“How long as it out there before we knew about it? That information today gives us a really strong sense it may not have been long,” she said.

Another piece of encouraging news came from wastewater testing, which showed COVID-19 was present in Auckland – but nowhere else in the country.

That could lead Ms Ardern to relax lockdown measures for outside Auckland.

Cabinet will consider whether to extend the lockdown – initially set for a week in Auckland and the Coromandel, and three days for the rest of New Zealand – on Friday.

COVID-19 researcher Shaun Hendy told Radio NZ his models “suggested about 100 cases out there”.

“We are dealing with a large cluster … we’re probably not looking at a situation where we’re going to be able to contain and eliminate this cluster. It’s going to take probably a couple of weeks.”

-AAP