The Ministry for Primary Industries seized 10,000 cockles from a group of illegal harvesters.

People on a family outing in North Canterbury collected more than 10,000 cockles breaching their maximum take of 600.

The four who have admitted breaching the amateur fishing regulations said they did not know that the daily limit for cockles was 150 for each person involved.

The prosecutor for the Ministry of Primary Industries, Grant Fletcher, told the Christchurch District Court: "This matter is the largest amount of illegally gathered cockles in the South Island and probably New Zealand."

A fifth person has also been charged and is due to appear in court on August 13.

Those who pleaded guilty before Judge Brian Callaghan were Xihou Lu, 45, a shop owner and cook, Xue Jin Mao, 53, a vegetable gardener, Wen Cai Mao, 22, a student, and Baoyi Mao, 24, a personal assistant.

The judge convicted and remanded them for sentence to August 20.

Defence counsel Craig Ruane said there had been talks with the MPI about some work the four would do to assist with community education on this issue.

"I want to get that put in place before sentencing takes place," he said.

He also wanted to discuss issues about the forfeiture of two vehicles used.

The offending is punishable by a fine or community work.

Mr Fletcher said the group went to Saltwater Creek, near the Ashley River mouth, on April 20.

Before they left, Baoiyi Mao had searched the Internet for the best places around Christchurch to gather shellfish.

"She had found information that suggested that the Saltwater Creek estuary on the Chinese-New Zealand website 'skykiwi' as a good place," he said.

The group parked near the river mouth, walked to the estuary, and gathered shellfish for about three hours before returning to their vehicles with nine sacks of cockles. They were joined briefly by three other people.

A member of the public was concerned about the quantity being gathered and their report led to Fisheries Officers arriving and stopping the vehicles.

The sacks were found to contain 10,548 cockles, and the vehicles were seized. The cockles were returned to the sea.

The four said they did not know about the daily limits. One said the group was on a family outing and the cockles were gathered for a party. Another said they took this number because there were "just so many".

One said, "he didn't count the cockles as everyone was just so happy", Mr Fletcher said.

The MPI says that cockles help filter water with each tidal cycle. This type of suspension feeder is described as being a key species in many aquatic ecosystems.