Spending by health boards on external consultants is costing between $10 million and $60m a year, and is poorly monitored, research shows.
The Otago Business School study covered DHBs' spending on outside consultants between 2012 and 2015.
Robin Gauld said the research showed many boards could not easily identify the extent to which they used outside consultants, or for what purpose.
He said the public might find that unsatisfactory given that health budgets were becoming more stretched.
"There ought to be some method for ensuring that there's a very valid reason for procuring an external consult and a framework for coming to that kind of conclusion.
"[And] that the services that are procured are reviewed in some sort of way to ensure that it is of a high quality given the amount that's being spent."
Professor Gauld said only six of the 20 DHBs had formal policies on engaging outside consultants.