A possum spotted floating on driftwood near Kapiti Island has boosted a hunch about how a stoat arrived on the predator-free island two years ago.

Fishermen Alan Wood, Mike Maybe and Joshua Morgan were fishing last month on the western side of Kapiti when they spotted the adult possum on a log about 60 metres from the island.

They saw the possum twice, first about 11am heading south with the outgoing tide and then again at 3.30pm heading north. Mr Wood took photographs and a film with his cellphone and passed them on to the Department of Conservation.

DOC spokesman Colin Giddy said it was the first evidence of a pest catching such a ride but conservation workers believed this could have been how a pregnant stoat arrived in 2010. Three stoats were caught last year and were believed to be related.

Monitoring and trapping was continuing but the prospect of a stowaway possum would not pose the same level of threat as a stoat to the island sanctuary.

Mr Giddy said the worst-case scenario would be if the possum were a female with a male joey, which meant there would be a breeding population after one to two years when the male reached sexual maturity.

However, it was unlikely the possum washed up on the island as staff had been scouring the beaches and could not find the distinctive wishbone-shaped log that would have been covered in possum scratches and even bite marks.

Possums were eradicated from Kapiti Island in the 1980s and it is home to some of the rarest wildlife in the country.