Director general of health Dr Ashley Bloomfield says he would have hoped the latest MIQ workers to have tested positive for Covid-19 had been vaccinated by now.
A third worker at the Grand Millennium managed isolation facility has tested positive for Covid-19.
The person was a colleague of the security guard, known as Case B, who returned a positive test last week. The pair worked a shift together over the Easter break.
Both workers are genomically linked to Case A - a cleaner from the facility who tested positive in March.
Bloomfield told Morning Report he is awaiting information on the vaccination status of the latest case but he "would have hoped that they would have been vaccinated by now".
He said 80 per cent of workers at the security company where both Case B and C are employed had been vaccinated.
"There was a clear expectation that people working in the frontline at the border do get vaccinated or there has to be other options looked at for them."
On three cases coming from the same managed isolation facility, Bloomfield said: "There's an issue."
"Last week when we got this case on Thursday we immediately got our MBIE teams and health teams together to work out what else is going on potentially in this hotel and we've convened the technical advisory group to give further advice there."
No further returnees are being hosted at the Grand Millennium Hotel, he said.
Case A, the cleaner who tested positive on March 21 is genomically linked to a returnee from March. Case B tested positive on April 8.
There is a long gap of time between Case A and B testing positive, Bloomfield said, and his team is investigating whether there was someone who was an intermediary between the two.
"That's all being looked at through CCTV footage and just whether both the cleaner (Case A) and the security guard (Case B) were in the same place at the same time."
It seems unlikely there would be an unknown part this, Bloomfield said.
"We're looking at all of those things."
There are 15 total close contacts of Case B still isolating. They will have two tests during their 14-day isolation stay.
There are four new locations of interest connected to the cases, all in Mount Roskill.