Key Points:

Independent MP Gordon Copeland has written to the Commerce Commission asking for an investigation into the price of milk in New Zealand.

Mr Copeland last month released figures showing that at $3.25 for two litres, milk was more expensive here than in some overseas countries including Australia, Britain, United States and Canada.

He said today he believed New Zealand prices should be significantly below those charged in milk importing countries when account was taken of freight, insurance and tariffs.

"In any event, since it is clear that the rapid increase in the price of milk is now putting a great deal of pressure on family budgets, I believe that an approved mechanism should exist to ensure that the New Zealand consumer is paying a fair price for milk," he said.

"The goal of the investigation would be two fold - firstly to ensure that milk is currently correctly priced and secondly that future price increases are able to be justified against objective criteria and in a transparent manner."

When Mr Copeland first complained about the price of milk, Fonterra said it had a responsibility to its shareholders to get the best price.

"What our customers pay is based on what that milk is worth on the international market," Fonterra spokeswoman Hilary Marett said on March 17.

Consumers Institute chief executive Sue Chetwin said at the time she did not know whether Mr Copeland's figures were correct but she would be worried if there were high retail margins involved.

"We have just started an investigation of our own into how Fonterra operates," she said.