The mystery cluster of Covid 19 cases in Auckland linked to the infection of 35 people is believed to be a stag party.

Director-General of Health Ashley Bloomfield made the announcement at his press conference this afternoon, after days of speculation about what the cluster was, and why it hadn't been named.

It is currently the fourth biggest cluster of Covid 19 cases of the 15 clusters in the country.

The Ministry of Health defines a cluster as a group of Covid-19 cases who are linked together because they have been in the same place together.

Until now, officials would only say the stag cluster was linked to a "private party to celebrate an event", and further information was behind withheld for privacy reasons.

Previously, there were two unnamed clusters, but on Sunday an Auckland facility caring for people with intellectual disabilities was identified as the workplace at the centre of the other.

The stag party case is being treated differently to other clusters, such as a wedding at Bluff in Southland, a St Patrick's Day party at the Redoubt Bar and Eatery in Matamata and the World Hereford Conference in Queenstown, which have been made public.

The Herald has lodged an urgent complaint under the Official Information to the Ombudsman about why some information is being withheld. This includes information about other Auckland cases, and whether "index patients" - the first person in the cluster to get the virus - have been identified.

On Sunday, Director-General of Health Ashley Bloomfield defended the lack of public information released about the Auckland party cluster, saying it was a "private event" which was "well-bounded" and everyone involved had been contacted.

On Saturday, the Director of Public Health Dr Caroline McElnay said the event was held before the lockdown.

"There is ongoing transmission happening from some of the people who were at the function ... that infection has passed within household bubbles during lockdown."

McElnay was unable to give more details, other than that the event was not linked to a workplace.

"What we are seeing with these private functions - and we see that particularly with the Bluff wedding function - is that social events, like weddings, like parties and other social events, really act as a mixing bowl for infection to be spread.

"We are seeing that in New Zealand and other parts of the world as well."

The largest cluster in the country remains the Bluff wedding, with 86 cases. Marist College has 85 and the cluster linked to a St Patrick's Day party at a Matamata bar has 71.

The clusters are now the centre of the spread in New Zealand, as overseas transmission declines.

Today there were 17 new cases of Covid 19, and 1366 cases in total. Four people died overnight, one in Wellington and three from the Rosewood cluster in Christchurch, bringing the death toll to nine.