Health officials are investigating whether there is an unknown person linking the three latest Covid-19 cases.

A third worker at the Grand Millennium managed isolation facility in Auckland has now contracted the virus.

They worked directly with the security guard who tested positive last week. That guard has been genomically linked to a cleaner who contracted the virus back in March, but it's not yet understood how that transmission happened.

Meriana Johnsen speaks to the Covid-19 pandemic modeller and Professor of Physics at the University of Auckland, Shaun Hendy.