The Government has spent $200,000 on an exercise simulating illegal immigrants arriving on New Zealand shores by boat, but can not say how big the threat actually is.
Although such incidents have not happened before, the Government says it needs to be prepared.
Earlier this year a group of Chinese asylum seekers left Malaysia bound for New Zealand, but arrived instead in Australia.
Minister of Customs Maurice Williamson says illegal immigrants are an unknown risk that needs to be dealt with.
The minister says the agencies taking part, which included Customs, Immigration and St John Ambulance, will review the exercise on Wednesday.