Immigration New Zealand is investigating whether its own staff were involved in a scam that has seen 279 Chinese students receive fraudulent visas to study in New Zealand in the past year.

Two agents in China are believed to be behind the fraudulent qualifications and bank statements provided to Immigration NZ in Beijing.

Immigration authorities want to speak with 231 of those students who are in the country.

The remaining 48 students who had been granted visas based on fraudulent documents have already left or never came to New Zealand.

The 231 in New Zealand are all studying at 20 English-language training providers in Auckland, Tertiary Education Minister Steven Joyce said.

Some of those education providers had 30 or 40 students on fraudulent visas, but most only had one or two.

There was no evidence those providers were complicit in the fraud, Mr Joyce said.

The fraud was uncovered in a regular audit by Immigration NZ about a month ago, and covered applications dating from July last year.

Labour Minister Kate Wilkinson told media the two agents involved were not accredited to Immigration NZ.

There was no evidence Immigration NZ officials were involved in corrupt activity or knew the visas were fraudulent, but Mr Joyce says the agency is still investigating whether "one or two Chinese nationals" it employs may have been involved.

"It's important to stress that that's not resolved at this point in time and that's for Immigration to work through," he said.

The 231 students could face deportation, although it was not yet clear whether they knew the documents used in their visas were fraudulent.

Mr Joyce says those who have already obtained qualifications from New Zealand will not necessarily be stripped of those.

"There are some that may actually have been quite entitled to obtain visas in New Zealand ... so every case has to be dealt with on a case-by-case basis."