The community Delta outbreak in Auckland has been found to be a "close match" to a recent returnee from Sydney.
Prime Minister Jacinda Ardern described it as a "significant" development.
She said it could focus the search for one or two missing links between the traveller and the community outbreak.
"Today I believe we have uncovered the piece of the puzzle we were looking for," Ardern said.
Eleven new cases of Covid-19 have been found in the community, bringing the total to 21.
Genome sequencing found the community cases "are a close match to a recent returnee from Sydney on a managed red zone return flight", Ardern said.
The person arrived on August 7, was tested on August 8 and returned a positive Covid-19 result on the 9th.
They were transferred from the Crowne Plaza to the Jet Park that day.
"They subsequently became unwell," Ardern said.
"This is a significant development, we can be fairly certain how and when the virus entered the country."
She said there was still one or two missing links.
Yesterday, the Government made mask use at essential services mandatory for both visitors and workers.
There are a number of locations of interest which are continuously being updated on the Ministry of Health's website.
Follow the latest developments on New Zealand's Delta outbreak on day two of Alert Level 4 lockdown.
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