Researchers are shocked at the speed with which mice over-ran an island in the Hauraki Gulf after just two were released in a controlled trial.

In five months the pair triggered a population of nearly 70 - demonstrating the danger pests can pose to endangered native species on our predator-free island sanctuaries.

University of Auckland researcher Helen Nathan tracked the population explosion on 6ha Te Haupa/Saddle Island, which had only just been cleared of pests before the study.

One male and one female were individually released on to the island.

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The numbers grew to 44 rodents within four months and then peaked at 68 five months after the release.

The population then stabilised and was exterminated at the study's end.

A surprise result from genetic analysis showed not all the mice trapped at the end were descended from the founding male and female.

An unrelated female - likely to have arrived on a visitor's boat - was first captured three months after the start of the invasion, and had produced offspring one month later.

Ms Nathan said the boom on the island - a scenic reserve where there were no threatened species considered at risk from the mice - was also what could be expected with other pests such as rats, ferrets and stoats.

The findings reinforced the need for ongoing monitoring for those islands that played crucial roles in preserving critically endangered species such as kakapo and kokako.