The number of anonymous tips to Immigration New Zealand has doubled in two years.

Almost 1800 complaints from unknown sources were recorded last year compared to fewer than 900 in 2016.

The figures, provided in an Official Information Act request, show in the first 10 days of this year alone Immigration New Zealand received 54 anonymous allegations.

Immigration New Zealand accepts complaints directly about fraud and exploitation, and also uses the Crimestopper freephone and online service.

The immigration compliance branch also asks for information from employers about immigrants who have left their job and may be in breach of their visa.

In the last financial year, 642 people were deported and another 2304 departed voluntarily, compared to 727 and 1437 respectively the year before.

Despite those increases, a memo last March to the Immigration Minister Iain Lees-Galloway suggested people living in New Zealand illegally were still on the rise.

It said the numbers of overstayers and increased fraud and exploitation work were overstretching its compliance officers.

The government set aside an extra $34 million in last year's Budget for Immigration New Zealand.