An investigation is underway into how a traveller made their way from Perth to New Zealand at the weekend while the Western Australian city was still in lockdown and quarantine-free travel was suspended.
Officials say the person arrived in Auckland from Perth on Sunday after travelling via Sydney.
Quarantine-free travel between New Zealand and Western Australia was temporarily suspended last week when Perth and nearby Peel entered a snap three-day lockdown, after community cases of COVID-19 were detected in the city.
"The New Zealand citizen who arrived back in New Zealand yesterday afternoon travelled on an Air New Zealand flight from Sydney to Auckland after flying domestically from Perth," Immigration New Zealand (INZ) border manager Peter Elms said.
"Due to the late booking of the Air New Zealand flight INZ no border alert was loaded before the flight departed Sydney.
"INZ subsequently became aware of the passenger's arrival in Auckland during routine passenger screening checks against the list of ineligible individuals. Steps were then taken to alert the Ministry of Health and secure a copy of his arrival card for contact tracing purposes."
In a statement, the Ministry of Health reiterated risk to the New Zealand public as a result of this remained low.
"Any person who enters New Zealand who is ineligible for quarantine-free travel is required to isolate for 14 days and is subject to penalties.
"The person arrived in Auckland and travelled on to Northland. Immigration's border operations became aware of this person's arrival during routine passenger screening checks after they arrived in Auckland."
"It serves as an important reminder that our public health response relies on accuracy and people fully complying with the safety measures that have been put in place by both countries.
"The actions of one person could jeopardise quarantine-free travel with Australia for everyone."
Anyone who has been in Perth or Peel since April 17 can check the locations of interest page in the Western Australia Department of Health site.
"New Zealand health officials remain in contact with their Australian counterparts and are closely monitoring the situation in Western Australia," the ministry said.
"All people who have been in Perth or Peel between April 17 and April 26 should watch for symptoms for 14 days after leaving these areas and if symptoms develop they should stay home and get a test," the ministry said.
Perth emerged from its lockdown overnight but quarantine-free travel between New Zealand and Western Australia remained suspended on Tuesday. The ministry said it would provide an update on travel restrictions later.
NZ case update
Eight new cases of COVID-19 have been confirmed in New Zealand's managed isolation facilities.
On Tuesday - 48 hours after NZ's last COVID-19 update - the ministry said the total number of active infections in New Zealand is now 36.
New Zealand's total number of confirmed COVID-19 cases is 2253.
Wastewater
A weak positive COVID-19 result has been detected in a wastewater sample from the eastern interceptor in central and eastern Auckland, the ministry has confirmed.
"It is the same area where ten recovered people have recently returned, after spending time in the Auckland quarantine facility," the ministry statement said.
"Seven of these people were released within 1-2 days prior to the sample being taken. Another sample is being taken today with results expected later this week."
The ministry said another sample is being taken on Tuesday with results expected later this week.
"We know that people who have recently had COVID-19 may continue to shed fragments of the virus for some weeks after they have recovered, without being infectious to others. Wastewater sampling can detect these fragments, which are not infectious and their presence is not considered a risk to the community.
"The recovered cases are no longer infectious and there is no evidence that the wastewater is infectious."
