The woman - identified by police - was reportedly the travel companion of the female confirmed Covid case, who returned two positive Covid tests.
The first test taken in Whangārei detected a small trace of the virus on October 4, and then three days later a second test on October 7, this time in Auckland, returned a positive result.
The Covid case - initially reported as an essential worker - reportedly used false documents to cross the alert level boundaries on October 2.
Covid-19 Response Minister Chris Hipkins said the confirmed case had been to several places in Northland - including Onerahi, Kensington, Paihia, Kawakawa, and Waipū.
The woman's actions saw the Government move Northland from alert level 2 to alert level 3 in a snap announcement on Friday night.
Hipkins previously said police were needed to track down the infected woman, as she did not provide sufficient information to enable her to be contacted with the results of her second positive test.
Prime Minister Jacinda Ardern told a Radio New Zealand reporter on Saturday that information about the two women's movements had been hindered by a lack of co-operation.
"As we've had information about their movements, we've released them. But this is a case where the individual, despite the involvement of the police, has not been forthcoming and that is why we took a very precautionary approach, and that was the right one."
"What we do know from video and CCTV footage is that the individual in question was travelling with a woman. We know that the place in which they were staying for that period of time has also confirmed that they were travelling with a woman."
She said the move to alert level 3 was necessary because "we have large gaps in what we know of their movements".
The woman who tested positive for Covid remains in an Auckland quarantine facility.
The MoH is continuing to urge Northlanders to get tested and vaccinated.
Northland testing centres open on Sunday are:
Whangarei – Rock and Roll car park, Pohe Island (9am–4pm), Kamo, 20 Winger Crescent (9am –4pm)
Kaikohe - Lindvart Park Pavillion, Penne Crescent (10am–2pm)
Kerikeri – 1 Sammaree Place (9am-4pm)
Kaitaia – Kaitaia Hospital, 29 Redan Road (9am-4pm)