Health officials should have been more up front after discovering five people could have been given saline instead of the vaccine, Covid-19 Response Minister Chris Hipkins says.
Hipkins has also admitted he was aware at least one more case of vaccine error - prompting revaccinations - occurred, if not more.
This morning RNZ revealed an investigation had begun after staff at the Highbrook vaccination centre last month realised there was an extra vial left over at the end of a day of 732 vaccinations.
It indicates people may have been given saline - a salt water solution - instead.
The news had opposition parties demanding more transparency from the government, saying it must explain what it knew, what it did not, and what it planned to do.
Under questioning at the Health Select Committee by ACT Party deputy leader Brooke Van Velden this afternoon, Hipkins said he was aware there was at least one other similar incident, after which people were called back to get a vaccine again.
He said he did not have the details on that case but it only affected a "couple of people", and he would have to find out from officials if there was more such cases.
'Nerve-racking' wait for information
At the government's daily Covid-19 briefing, Director-General of Health Dr Ashley Bloomfield revealed the error happened on 12 July.
The government was seeking advice on whether the 732 people who were vaccinated that day should receive another dose, he said.
"Everyone who was involved in that incident will receive a letter in the next 24 hours and there will be further followup discussions with them about what the next steps will be."
A man with an immune condition who was vaccinated at Highbrook in July told RNZ it was a relief to find out his vaccination was on a different day. He said the process at the vaccination centre had been great but he would have wanted to know immediately.
"Because of my vulnerable state I'd obviously want to know as soon as possible to make sure it's not me or make sure it's not my family being put at risk.
"Being in a lockdown situation anyway as a person in my situation ... you're quite at a heightened sense of anxiety but yeah it was a little nerve-racking this morning trying to figure out is it the day that I was there, and if so what are they gonna do ... am I gonna get a booster or a third shot".
"I guess it's just making sure that the people who are really at risk here like myself and other people going through treatments, or people who just can't have a vaccine, that they're treated as a priority and protected."
Bloomfield clarified at the briefing the decision to contact people was not made until after RNZ started making enquiries about it.
However it was always the intention to contact those people, he said, and they would have been regardless of whether the story had broken on RNZ today.
At the select committee later, Hipkins acknowledged the delay in making the information public was "regrettable".
"I think it would have been better if they had been more up front sooner after this happened," he said.
"As Dr Bloomfield indicated at the press conference today, they did intend to get in touch with people to let the know what had happened, they did intend for this information to be made public. I think it would have been better if that had happened earlier but he also indicated that there was still some disagreement as to what the most likely thing to have happened here would have been."
Hipkins said human error could never be fully guarded against and it would be unreasonable to do that, but the vaccine rollout had been safe and efficient.
"Our utilisation rates of the vaccine doses sits at about 99.5 percent when the world health organisation ... say to plan for a 10 percent vaccine wastage rate and we've got less than half a percent.
"That's a sign that the vaccine is being rolled out very safely."
Changes to be brought in to protect against further errors
Bloomfield said the team had gone back and looked again at record keeping around dose administration.
Changes had been brought in, he said, including more frequent checking of vials, and labelling of syringes at all large vaccination sites.
He said he was comfortable the ministry was going through a "very thorough" process.
"It may well be that some of the vaccinators were getting more than five doses out of a vial but that hadn't been recorded."
He said the record keeping at that point was up to the standard he would have expected.
"Yes, because every person that went through, the fact that htey got vaccinated was recorded on the Covid immunisation register so there was no issue with the record keeping at all."