All Marist College students are being asked to get tested for Covid-19 before returning to school - even if they've already been tested or have no symptoms.

The Catholic girls' high school is the epicentre of one of New Zealand's largest clusters of Covid-19.

The decision was made over the weekend, school principal Raechelle Taulu, board of trustees chairman Stephen Dallow and Dr Julia Peters of the Auckland Regional Public Health Service said in a letter to parents this morning.

"Although testing is voluntary, we strongly recommend that everyone at the school takes up this opportunity. The chance of detecting any new cases is very low. However, we are determined to take every precaution."

A question and answer sheet for parents raised the question of why more people were not tested earlier when this could have stopped the spread of the virus.'

"Previously the focus was on testing people who presented with Covid-19 symptoms," the sheet says.

"We now want to make sure that there is no virus still circulating amongst staff and students as they head back to school. It is possible that there is someone who has Covid-19 but has only mild or no symptoms."

The school would not be told the names of anyone who tested positive - privacy would be respected at all times, the letter to parents said.

"Testing all staff and students will give extra reassurances that when we do open the school gates, your daughters will be walking into an environment where every precaution has been taken to protect their safety."

Any student who was ill - even mildly - should stay away. That included girls who tested negative for Covid-19.

Parents were reminded that there could be new infections in future months and everyone should stay alert for Covid-19 symptoms including fever, cough, sore throat, runny nose, loss of smell or difficulty breathing.

Everyone was being provided with a testing voucher which was valid till the end of this week. The 14 community-based assessment centres in Auckland would be expecting girls from Marist and would test them on presentation of the voucher, even if they didn't have symptoms.

Next week the school board, ARPHS, DHBs and the Ministry of Education will decide whether to reopen Marist. Online learning is continuing for all students during level 3.

The school also told parents there had been 46 cases directly associated with Marist. The Ministry of Health's website says there are 94 cases that have arisen from the "Marist cluster", with 86 of those recovered. The origin of the cluster is listed as unknown and transmission is considered to be ongoing.

The test results would be reviewed and any positives assessed - particularly whether those people were still infectious. While nobody with a positive result would be allowed back to school till they had been cleared by ARPHS, "if there are a number of people who are considered infectious, the school will remain closed", the school said.

Marist had only had one new case in the last 14 days, parents were told. It was thought to be unlikely that there would be new positive cases - if all students and staff had abided by the close-contact quarantine and then followed the lockdown rules.