Prime Minister Jacinda Ardern and Director-General of Health Dr Ashley Bloomfield revealed that whole genome sequencing has found that current positive cases are a close match to a recent returnee from Sydney.
The individual arrived on a managed red-zone flight on August 7 and was at the Crowne Plaza managed isolation and quarantine facility, before being moved to Jet Park.
But they were transferred to Middlemore Hospital after falling ill.
Public health officials are now focusing on workers at the Crowne Plaza and Jet Park quarantine facility. Middlemore Hospital is not under investigation at this stage.
There are now 21 cases of COVID in the community, health officials announced at Thursday's briefing.
The Pfizer vaccine will also be available for 12-15 year olds and parents will now be able to book in their children's vaccinations at the same time as their own from September 1.
The Prime Minister confirmed that 195,000 people booked a vaccine yesterday.
Cabinet will meet on Friday morning for an update on the current alert level settings.
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