Joint Head of Managed Isolation and Quarantine Brigadier Jim Bliss and Director-General of Health Dr Ashley Bloomfield are providing an update on the situation.
Recommendations for managed isolation and quarantine (MIQ) facilities have been made as a result of the reviews into the cases from March and April.
The reviews were undertaken by MIQ and the Ministry of Health. There were no community cases as a result of these incidents.
However, they prompted authorities to empty out both facilities and complete on-site assessments of the ventilation systems.
In a statement, Bliss said the MIQ system was continuously improving and evolving.
"The two facilities will remain unoccupied until such time as the necessary work has been completed.
"A programme of extensive reviews and remediation of ventilation systems across all managed isolation facilities is underway.
"Remediation work at the Grand Mercure is almost complete. An extensive assessment of the Grand Millennium's ventilation system has been done and a remediation plan is being developed."
Dr Bloomfield said returnees to New Zealand and the wider community could feel confident in the MIQ system, adding there was multiple layers of protection.
"As part of this process, the Ministry of Health undertakes regular infection prevention and control audits of the MIQ facilities, and any recommendations are actioned," Bloomfield said.
"The reviewers themselves noted that, whilst the reviews focused on outlining necessary improvements as a result of the incidents, it is in fact the success of the wider MIQ system that has been integral to the nation's success in keeping Covid-19 largely out of our communities."
Grand Millennium case review findings and report
The Grand Millennium case involved three workers testing positive for Covid-19 in March, among them a security worker (Case B) who was subsequently found not to have been tested for a number of months.
The review could not conclude with certainty how the cases become infected, but said the most plausible hypothesis was that Case A became infected from the Index Case via aerosol transmission in a hallway, and between Case B and Case C by direct exposure from two workers on the same shift.
The review made six recommendations, including improving barriers to staff testing and vaccination, and continuing to improve data management systems.
The Ministry of Business, Innovation and Employment (MBIE), in charge of MIQ, said all of those recommendations were either underway or completed.
KPMG was also commissioned by MBIE to review what led to the security worker not being tested for Covid-19 for a number of months.
The report stated Case B falsely stated they had undergone nine tests between 11 December 2020 and 24 March 2021 to their employer, First Security.
It also noted that First Security were made aware of Case B's non-compliance with testing on 8 April 2021.
Since then, First Security has developed systems and processes to keep and maintain records of border workforce testing in accordance with the Required Testing Order and Health and Safety at Work Act, KPMG's report stated.
Eight opportunities for improvement for MIQ were made, and MBIE said all were either underway or complete.
After the case, the Border Worker Testing Register (BWTR) was made mandatory from 27 April 2021.
While the BWTR was being rolled out at the time of the incident, MBIE said the systems were not in a position to support active management of staff testing compliance.
The ministry said that system was now working well, with MIQ moving from a high-trust model to a model where employee, employer and MIQ now sharing a greater responsibility for ensuring compliance.
Grand Mercure case review findings
The Grand Mercure case review looked into two incidents, the first between two returnees and the second with a returnee who tested positive while out for an exercise but was put on a bus with other isolating returnees.
In the first incident, the cases were genomically linked, which the review said strongly indicated in-facility transmission.
The review found that while aerosol transmission via the ventilation system seemed unlikely, it was the most plausible transmission pathway between the index and secondary case.
It stated the risk of downward airflow between rooms appeared to be unique to the Grand Mercure Auckland.
However, it said the overall risk of in-facility transmission to the returnees at the Grand Mercure was low, considering infection prevention and control measures.
The second incident, involving the same secondary case who was allowed back on a bus with other returnees after getting a positive test result, resulted in 14 others being considered close contacts and having to stay another 14 days in managed isolation.
The review found there was a breach in bus protocols and non-compliance with the Standard Operating Procedures. The case's blue wrist band - indicating they could go out for a walk - was not removed.
MBIE said it had since implemented a number of recommendations on improving managed isolation walks.
The review made five recommendations in total, and MBIE said all but one were either underway or completed.
The remaining recommendation is being considered by the Ministry of Health Technical Advisory Group.
