A finance company implicated in student visa fraud in Sri Lanka has been providing information for Immigration New Zealand applications since 2012.
Investigations have been launched into the Sri Lanka-based finance company, local education agents and possibly students themselves.
But immigration officials said they were hamstrung when it came to past visas so would not be reviewing all applications.
National said that was outrageous, and showed the coalition government is going soft on an issue it talked up during the election campaign.
Immigration fraud is nothing new but officials are investigating what appears to be extensive and organised fraud.
It relates to a finance company falsifying documents showing students have the required $15,000 in the bank for a student visa.
Immigration New Zealand figures show just over 6500 student visas from Sri Lanka have been approved since 2012.
Stephen Dunstan from Immigration New Zealand said the company had been involved in applications since 2012 but ramped up last year.
"It looks like they became quite big players in 2017... there were applications prior to that but we haven't got evidence that they were involved in fraudulent information at that time."
Their best estimate, he said, was that about 400 applications from 2017 may be questionable.
Immigration Minister Iain Lees-Galloway said officials would scrutinise any student already here if they tried to apply, but otherwise were somewhat hamstrung as the company was supposed to be providing the authenticating information.
Former minister and National's immigration spokesperson Michael Woodhouse said sampling of past years should be done so officials know what they're dealing with.
Immigration New Zealand said the Sri Lankan company and the international education market had been told any application with information from the company as proof of funds would automatically be rejected.