The ACT Party says it remains to be seen if the benefits of a new medical school at Waikato University stack up.
The comments came despite expert analysis that found they did add up.
Papers show leader David Seymour told the government in August he was "dissatisfied" with an analysis he said skipped over key costs.
He reiterated those comments on Thursday.
"It is important we get a proper evaluation to make sure the government is pursuing the most effective policy," Seymour said, after the Ministry of Health released his letter in August, along with the cost-benefit documents.
"It's possible this option stacks up, or we might determine that funding is more effectively spent on policies like increasing GP capitation or expanding other medical schools."
The initial analysis scored a new school ahead of expanded training at the existing schools in Auckland and Otago, particularly in relation to rural needs.
More analysis would occur in a detailed business case, which was expected to be released next year.
Under ACT and National's coalition agreement, a full cost-benefit analysis (CBA) needed to be completed before any binding agreement was made.
"We are currently working through a robust cost-benefit analysis for a potential third school in Waikato to continue to grow the number of New Zealand-trained and culturally competent doctors," Minister of Health Shane Reti said in a statement.
In September, however, Reti said Cabinet had considered the "completed" cost-benefit analysis, which had "been found to provide confidence for the project to progress to the next stage".
The CBA provided assurance the proposal "would achieve the government's objectives and provide significant benefits," Reti said in September.
The early CBA said that out of three options looked at, a new medical school was the most expensive but was also expected to reap the most benefits - though noting this was "very sensitive to key assumptions".
Seymour challenged those assumptions in a letter to Reti in August.
"I do not have confidence in this finding," he wrote.
He wanted the analysis to do more about gauging the cost of producing graduates who did not become GPs.
"The omission of the benefits and costs of specialist doctors [who will represent more than 80% of the graduates of the proposed Waikato medical school] materially alters the conclusions" and meant the coalition agreement condition was not met, he said.
He had his own analysis done, producing a chart that showed under many scenarios Waikato was not the best option for producing and retaining GPs than the other medical schools.
The ministry's consultant, Sapere, told the ministry's health economist the very next day that all the options scored high for benefits, but Waikato especially so.
"The evidence used in the cost benefit analysis is the best available to us, is highly relevant, generalisable to New Zealand, and is recent," its report said.
Reti wrote to Seymour, saying Sapere had relied on experts, and gave a solid defence of its CBA, but that it would be "very complex" to go further.
At the stage this had got to in September, the best practice guidelines from Treasury suggested that less than half of the economic case for the options would be expected to be done; the rest of the testing of that would follow, Reti told the ACT leader.
Going with Waikato did not rule out other moves to boost medical education or the workforce, he added.
Last year, National said a $380m rurally focused school to open in 2027, taking 120 students a year with any type of undergraduate degree, was an "absolute priority".
"New Zealand urgently needs to grow its medical workforce," Reti said today.
"As minister, I've previously said that we very much value the work of the Otago and Auckland medical schools. That doesn't change."
Seymour said on Thursday he looked forward to seeing the "robust" cost-benefit analysis now underway.
"I intend to honour the coalition agreement with National, which means no binding agreement will be made until a full cost-benefit analysis is considered."
Green Party public services spokesperson Francisco Hernandez said the documents blanked out the specific costs breakdown, making it impossible to judge the robustness of the CBA, which had also not assessed the risks of building a new medical school from scratch.
"The documents ... shows how flimsy the foundation is for the proposed medical school and the tensions within government over this issue," he said.
"Even if you were to take this report at face value, the Waikato Medical School proposal only achieves a BCR [benefit-cost] ratio of 2.7, which is only 0.1 higher than the alternative proposal."
He called on the government to release the full costings.
