No one was drug tested before a pilot Mongrel Mob-led rehabilitation programme, which later received $2.75 million of government funding because it showed "signs of success".
National's deputy leader and health spokesman, Shane Reti, says it calls into question the Prime Minister's claims of the controversial programme's "success".
Kahukura is the controversial marae-based rehabilitation programme being run by Hard2Reach, to support whanau and communities following an increase of suicides and homicides in Hawke's Bay-based Mongrel Mob members.
Reti said written ministerial answers sent in reply to his questions show "no hard evidence" that participants on a pilot of the programme which ran last year were actually using methamphetamine.
Prime Minister Jacinda Ardern in July defended funding of the programme, after it was revealed by Hawke's Bay Today, saying the pilot had shown "signs of success", with strong court order compliance and drug testing results.
"We either have to make a decision to fund programmes which, yes, involve people with criminal history but we are determined to address their methamphetamine addiction, or we exclude people with criminal histories from meth addiction programmes," she earlier said.
Reti, himself a former general practioner, wrote to Health Minister Andrew Little asking if laboratory testing was undertaken early on to show that participants were actively using meth.
The reply stated it was not, and that the testing used was a screen with short-term sensitivity only.
"Negative tests are not unexpected as participants were expected to be committed to giving up methamphetamine and may have already begun this process," it said.
Reti said the lack of testing was one of several problems with the pilot which served as an "anchor" for the application for funding from the Proceeds of Crime Fund.
"There was no methamphetamine test before the trial started to act as a baseline and confirm that these 10 people were actually taking methamphetamine," Reti said.
"History taking is not enough for a trial like this as shown by the fact that during the rest of the pilot lab tests were used."
He said the fact that interventions may have already been occurring, also made it difficult to prove successful outcomes were the result of the pilot programme.
Subsequent urine testing for meth was undertaken during weeks two, three, four, and six of the pilot programme, the majority of which was undertaken by the project team leader on their own, answers sent to Reti stated.
Reti said he was concerned that testing during the pilot was not undertaken by an independent lab, describing it as a "conflict of interest".
A Ministry of Health spokesperson said the ministry "did not have oversight" of the pilot programme but understood participants were assessed prior to entering the programme.
For the current courses, being funded directly, participants are vetted using a standard and widely-used clinical assessment prior to entering the course, they said.
"Participants who are then accepted on to the course are drug tested at regular and random intervals to determine progress.
"As with many treatment programmes around the country, it is common for trained staff to administer a range of clinical assessments including drug testing for participants."
Questions put to Ardern's office were answered by a government spokesperson, who said the programme was supported on the advice of the MOH, Corrections, Police and MSD.
They said it was based on a programme that was run in 2010 as part of the then National government's methamphetamine action plan.
"It is very much focused on trying to reach those who may not be captured by other programmes to address meth addiction and the crime that often results from meth addiction."