The Ministry for Primary Industries (MPI) is not commenting on the first independent milk test results from Taranaki's so-called landfarms, where oil industry waste is disposed of.
The first tests are back from overseas.
A ministry spokesperson said there were no food safety issues at the levels detected but would not say what it was that had been detected in the milk.
Green Party MP Gareth Hughes called on the ministry to release the results.
"If they have detected oil and gas residues I think that would be concerning even if that's below food safety issues," Mr Hughes said.
"We've seen MPI in the past sit on data like this for months as they work out PR strategies. I think it's important that they take the opportunity to release this information as fast as possible."
Climate Justice Taranaki researcher Catherine Cheung said it was not on for the ministry to say there were no food safety issues while not saying what it had found.
"It's not good enough to say there are no food safety issues - the public has the right to know what's in it," she said.
"It could be hydrocarbons from the formations, it could be heavy metals, fracking chemicals - I don't know if they won't say what it is. I think the main point is oil and milk don't mix and they shouldn't be taking any more milk from landfarms."
Food Safety Minister Nikki Kaye said food safety was the primary responsibility of the food processor under the Animal Products Act and that the ministry's preliminary results indicated there was no health and safety issue.
However, she would not say what had been found in the milk.