The small reduction in cost at supermarkets does not match the halving of global milk prices since 2011.

When the global market for milk spiked three years ago, New Zealanders were told they must be prepared to pay the international price. High world prices were good news for this country, and we should be happy to pay no less, or more, than the product was worth.

But what when the cycle takes a sharp downward turn and is not expected to start showing signs of recovery until next year? Logically, the cost of milk at the local supermarket should reflect the depressed international picture.

That was not the case, the Labour Party's David Shearer insisted this week. Global milk prices had halved since 2011 and farmers were getting much less at the farm gate, he said. But the cost at the checkout did not reflect that. Mr Shearer had a point.

According to Statistics NZ, the average price for two litres of standard milk was $3.67 as the cycle peaked. It has fallen, but only, according to a survey of supermarkets, to a current low of $3.09.

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Indeed, consumers in Britain and Australia are paying less for milk. This, the Prime Minister said, was the result of price wars in those countries. He did not explain why there should not be a similar competition between supermarkets here.

Fonterra, for its part, says local pushback meant the peak domestic price in 2011 did not reflect the full extent of the global spike. That may be so, but its local consumers are due more compelling evidence that international price movements work both ways.