Victorian homes are being invaded by black Portuguese millipedes revelling in ideal conditions created by recent rains.

The critters are being seen in their largest numbers in eight years, says Melbourne Zoo's invertebrates specialist, Patrick Honan.

The recent wet weather had created an abundance of food for the imported pest, which was accidentally introduced to Adelaide in 1953 and has now spread across southern Australia.

"They increase in numbers every autumn and every spring across most of southern Australia, but in Victoria this year we have had a particularly wet patch and that really encourages them. This would be the worst we have seen for seven to eight years," Mr Honan said.

He said the millipedes, which are smooth and cylindrical and between 2-4.5cm long, secreted the chemical hydrogen cyanide when they were disturbed or squashed.