"A new programme is simply not in the national interest and taxpayers will have to pay for what is an ill-considered and expensive folly," said Professor John Fraser, dean of the Faculty of Medical and Health Sciences.
"The existing programmes at Auckland and Otago are already meeting the needs for growth of doctors for New Zealand."
Medical student numbers are growing as part of the Government's long-term plan to increase the supply of doctors, the university said. By 2020 New Zealand is on course to produce 570 graduates annually.
Professor Fraser also hit out at Waikato University's argument that the existing medical programmes based at Auckland and Otago were not producing the right kinds of doctors, saying it was "simply not true and reflects a naive assumption that careful planning has not occurred to address the workforce shortage they purport to address".
"Evidence indicates that a bespoke programme providing an alternative education path specifically to rural general practice, does not generate larger numbers of graduates committed to being a rural GP.
"What international evidence does indicate is that selecting medical students from rural and regional backgrounds and providing them with a quality rural experience encourages them to return to those regions to work as doctors," he said.
The University of Auckland has had a clinical campus in Waikato for 20 years, with other clinical campuses for training medical students at Whangarei, Whakatane, Tauranga and Taranaki hospitals.
Expansion of the programmes at Tauranga and Taranaki is planned for 2017 and 2018.
University of Auckland said it was concerned the proposal for a new medical school would end a long-standing partnership with Waikato DHB and would "effectively displace 150 medical students now training at Waikato Hospital, with nowhere for them to go".
"Introducing another 240 medical students into a medical training system that is already under severe pressure to provide training places can only be considered foolhardy," Professor Fraser said.
"The cost of establishing a medical programme should not be underestimated, so this should be seriously considered only when a clear national need has been argued."
'Far-reaching consequences'
Professor Peter Crampton, Pro Vice-Chancellor, Health Sciences at the University of Otago, said the institute was also opposed to a third medical school.
He understood it "is the Government's prerogative to make such decisions", he said, but expected to be consulted on any proposal, describing it as a "serious decision with far-reaching consequences for New Zealand".
"A third school would be disruptive for Otago and Auckland medical schools because the proposed 240 new medical students at Waikato would necessarily take over existing training capacity," he said.
"A third school could also have serious consequences for the medical workforce with issues related to bottlenecks in vocational training and distribution problems.
"For example, we know from our own experience and from extensive experience in Australia, where there are now 19 medical schools, that increasing the number of graduates does not in any way guarantee that those graduates will choose to work in high-needs areas."
Since National came into power, there had been a steady increase in the number of medical students being trained at Otago and Auckland, to the point where New Zealand will soon be graduating close to 200 more doctors per annum than was the case a decade ago, Professor Crampton said - around 290 of those from Otago.
"This increase, along with a return of NZ doctors from Australia and a small number of New Zealanders completing medical degrees in Australia, is likely sufficient to meet NZ's doctor workforce needs for the foreseeable future."
He added: "We already have sufficient Māori students in our medical programme to increase the number of Māori doctors in this country by over 50 per cent over the next five years."
More doctors needed - Waikato
Earlier today Vice-Chancellor of the University of Waikato, Professor Neil Quigley, said New Zealand was not currently producing enough doctors to meet future demand.
"We need more doctors, I think that's clear," he told the Mike Hosking Breakfast show on Newstalk ZB.
"But of course part of the pressure in the system is the doctors we're training don't necessarily choose the specialities or want to work in the geographical locations where we're most short of people.
"So that's a key part of what we're trying to address as well."
Professor Quigley added: "Our population is growing very rapidly, and particularly outside of the main centres many of our general practitioners and the surgeons working in provincial hospitals are coming up to retirement age over the next decade and so it looks as though, on the current track, we really need a lot more doctors, much more than the two existing medical schools can supply."
Part of the university's proposal would include setting up community medical education centres throughout the central North Island, which would provide clinical placements for students and "ensure greater engagement with community medicine".
If the proposal gets the go-ahead from the Government, it would likely be 2020 before the first students could start classes at the new medical school, he said.