Nearly half of the 11,000 migrant exploitation and fraud tipoffs sent to Immigration New Zealand were not investigated in the six years to 2017.

From the 5700 cases that were investigated, less than 150 prosecutions were made.

Almost 500 cases, some four-years-old, remained open, according to figures released under the Official Information Act.

SCOTT HAMMOND/STUFF Immigration Minister Iain Lees- Galloway says immigration prosecution figures are eye-opening.

Immigration Minister Iain Lees-Galloway said seeing the numbers for the first time last year opened his eyes to the challenges immigration enforcement faced.

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"I am even more aware now of the situation we have inherited and the need to work quickly to address it."

SUPPLIED Immigration New Zealand assistant general manager Peter Devoy says his staff can only make so many prosecutions.

Cases that were investigated by Immigration were mostly over migrant worker exploitation, people smuggling, forged applications and partnership visas.

Immigration New Zealand assistant general manager Peter Devoy said he did not expect the revealing of Immigration's investigation and prosecution rate history to dent confidence in the agency.

"We are never going to get to the situation of being able to investigate everything. That is going to be a fact. We have to triage the work and look at what we can do."

In 2016, Immigration received 1000 more allegations than the year prior. The spike in volume resulted in 1363 investigations, the highest number in years.

The investigations led to 22 people being convicted last year, 10 fewer than in 2015.

Devoy said only so many prosecutions could be made.

"Prosecution takes a lot of time, effort, the high-end cases are resource intensive and they are time intensive."

Immigration had 64 compliance and investigation staff in June last year, 20 more than it had in 2012.

More than 5250 complaints made to the agency were not investigated between 2011 and 2016.

Devoy said it was often because allegations were made anonymously and lacked detail for staff to follow up.

"We get a whole range of reports. Any number of these reports are anonymous and quite often, while we might be slightly aware of a situation, identifying anything which might be evidential from the report and being able to follow that up is difficult."

The agency's focus was to crackdown on people trafficking, he said.

"We will take those matters forward and we will investigate them."

Although prosecuting people traffickers was tricky. It had to be approved by the attorney-general, he said.

Immigration worked with Police and labour inspectors on hundreds of deportation and civil proceedings.

Lees-Galloway said changes to the immigration system would be made.

His priority was to foster a culture where migrant worker and student exploitation was not tolerated, he said.

"Everyone needs to know that we won't allow the exploitation of migrant workers here."

He would not discuss Immigration resources and funding before this year's Budget.