The two mothers in the Auckland COVID-19 cluster breached lockdown rules by going for a walk during alert level 3 last month.
In a press conference on Monday, Prime Minister Jacinda Ardern said one of the mothers had been told to self-isolate.
Instead, she went for a walk with another mother, who tested positive for COVID-19 on Saturday.
The first mother was a parent of case D, a student at Papatoetoe High School who had tested positive for the virus on February 17.
The other mother (case N) tested positive on February 28 after her son (case M) returned a positive case.
There are fears case M and case N were infectious for as long as a week out of isolation.
Ardern said contract tracers have been continually following up with families of those tested positive.
"Anytime anyone is positive, our contact tracers sit down and take a comprehensive interview of that individual about all of the contact they've had," she said.
"That will often happen across multiple interviews to help someone spend a bit of time reflecting what they've done, go through their eftpos data, phone records to identify their movements."
"With that first household - remember it was a classmate - and then family members - and it was one of the family members who tested positive.
"They were interviewed but did not disclose the contact with this follow-up family," Ardern said.