Whānau Ora providers have told the High Court the health ministry's refusal to hand over the vaccination information of Māori is inconsistent and a breach of the Treaty of Waitangi.
But the Crown said to do so would be a serious breach of privacy, and could actually derail the vaccination rollout for Māori.
The Whānau Ora Commissioning Agency, which represents providers in the North Island, wants data for all Māori held by the ministry - including phone numbers and addresses - so they can tailor programmes to boost Māori rates.
Māori are the worst-affected by the Delta outbreak, making up more than half of all new cases and rapidly rising. Māori are also well behind in the rollout by some 20 percent.
The Ministry of Health will only release some data, that of people who are registered with or have used a Whānau Ora provider before. It also agreed to release other data that is anonymised.
But in the High Court at Wellington on Tuesday, the Whānau Ora Commissioning Agency said that level of information would be largely useless for really hard to reach communities.
Its lawyers argued the full data set would allow specific vaccination programmes involving phone calls or visits by people who are engaged with these communities on day-to-day basis.
One of its key arguments was that the health ministry has given the same information it is asking for to Healthline for its outreach campaigns.
Whānau Ora providers also receive similar data for other campaigns, it said, like measles, mumps and rubella vaccines. They argued both of these were a glaring inconsistency.
The Crown, however, argued that releasing Māori data would be a breach of privacy.
It said the health ministry was taking a wait and see approach, and vaccination rates for Māori were improving without it, pointing out there have been 140,000 Māori who have had a jab since the challenge was filed.
It would only release the data as a last resort if they really hit a wall. But when asked what point that might be, lawyers said they did not have one.
The Whānau Ora Commissioning Agency argued this was akin to allowing the issue to percolate when the need to vaccinate is now.
The Crown also said it was concerned that releasing the data could further undermine efforts in a community that is already sceptical of the state - releasing people's data could just add to this distrust.
Whānau Ora Commissioning agency also argued that the Crown is obligated to facilitate a kaupapa Māori programme and to ensure equitable outcomes.
Tino Rangatiratanga provides that Māori providers should be fulfilled and given full access, they said.
There was no dispute that the Crown had an obligation under the treaty as an overarching principle, but there was discussion about how it is applied in this instance.
Justice Gwyn reserved her decision, but noted the urgency of the case and said it would be released as soon as possible.