A supermarket near Auckland Airport is the last stop in New Zealand for many Chinese visitors as they stock up on New Zealand-made baby formula.

The Herald revealed at the weekend that some supermarkets were selling baby formula worth up to $170,000 in bulk to Chinese exporters while restricting ordinary shoppers to as little as three cans at a time.

The formula is sold at a profit to Chinese parents worried about contamination in their locally-made formula, and shortages have deprived many New Zealand mothers.

Shoppers at the Countdown supermarket near Auckland Airport said busloads of visitors would arrive weekly to buy formula.

Led by a tour guide, groups of about 40 tourists took their shopping baskets straight to the baby section and "literally stripped the shelves of all baby formula within minutes", said a regular shopper, Steve Marshall.

"Luckily for me my daughter has just got off formula, but I feel sorry for anyone living nearby that would need to make an emergency trip to the supermarket," Mr Marshall said.

The run has made it difficult for some Auckland residents to get their hands on formula.

Erena, an Auckland mother, said she visited seven shops looking for the Karicare brand of baby formula but failed to find any.

"I went to The Warehouse, Countdown and Pak'nSave in four different suburbs," she said.

"I'm lucky I have a car and petrol to run around looking for it, but what about others who don't have the means to carry out a mission to find it?"

Corinne Ola, of Mt Albert, mother of 3-month-old Arianna, said her local supermarket had been sold out for three weeks and she had to drive to Mangere for supplies.

When she finally found the local shop stocked with about 10 cans last Tuesday, it was letting customers buy more formula than its two-can limit for $3 extra a can, she said.

"It's shocking. I believe if the limit is two per person, they should enforce it so we don't suffer."

The traders of baby formula, who operate online as companies registered with Inland Revenue, also take GST refunds on their purchases.

Their large bulk purchases can entitle them to thousands of dollars in returns.

A source said this was key to how the traders made money.

"The GST they claim from IRD can be a lot more than the profit they make on selling the formula."

Some traders received as much as $60,000 from IRD every month, the source said.

The Ministry of Agriculture and Forestry said it would investigate the bulk buying, which constituted commercial operations and required export certificates.