Counties and Waikato DHBs are scrambling to work out how far Covid-19 has spread among its communities after three positive cases in Whakatīwai.

The first case was a prisoner bailed from Auckland to the small Firth of Thames settlement.

Two children who he shares a home with in Whakatīwai have since tested positive, resulting in a Covid pop-up testing site being set up at Wharekawa Marae, near Kaiaua.

Waikato DHB Chief Executive Dr Kevin Snee this afternoon said all tests would be fast tracked and the results could be expected late tonight.

Experts from Waikato DHB and Auckland Regional Public Health were now "working rapidly to confirm whether there had been any further transmission".

"Our advice to the local community is to exercise an abundance of caution at this stage.

"If you are able to stay home, please do. For those who do need to go out, please take extra care to wear a mask, practise social distancing, and wash your hands regularly."

The DHB was this afternoon putting in place a restricted visitor policy from 2pm and services across all hospitals reduced to acute and urgent care only from tomorrow, with outpatient clinics utilising virtual and phone consults wherever possible.

Thames Hospital began surveillance testing of all incoming patients last night.

"These are difficult decisions to make as they have a significant impact.

"However, the decision has been made to take a cautious approach, prioritising the safety of our patients and staff. Once we have a better understanding of whether there has been further community transmission, we will review this decision.

"The DHB will be contacting all affected individuals to discuss options for treatment and/or deferral if required and we apologise for any inconvenience or distress this disruption may cause."

The Ministry of Health today noted that the three cases recorded in Whakatīwai fall within the Counties Mānukau DHB catchment and therefore are being counted in the Auckland total.

Following confirmation of the three cases in northern Hauraki in the Waikato, science agency ESR is currently sampling wastewater from Kawakawa Bay and Pukekohe.

Samples are being collected from three sites in the region.

In addition, new samples are being collected from Cambridge, Ngatea, and Paeroa. Results are expected later in the week.

A drive-through testing centre at Wharekawa Marae had been busy today with vehicles lining up from 8am, for its 9pm opening.

More than 100 had been through by 11am.

Designated GP practices across the Waikato and the community testing centre at Founders Theatre in Hamilton also had testing capacity.

The impact from the cases at Mangatangi School could be widespread as people from all around the area including Waitakaruru, Maramarua and Mangatawhiri attended the school.

People living in surrounding areas also travelled as far as Auckland, Huntly and Thames to work.

A local woman waiting to be tested this morning worked at Middlemore Hospital, while her partner worked at the Huntly coal mine.

Mangatangi and Kaiaua schools were closed today.

Hauraki Plains College principal Ngaire Harris said there had so far been no confirmed link between the college and Mangatangi School.

"We sought advice both from the Ministry of Education and the Ministry of Health. Under that advice, the school is not a location of interest and is open for instruction but taking care that everyone is following the safety guidance currently in place," she said.