The snake that slithered out of an imported car at a car yard in Auckland was missed by a Japanese company accredited by the Ministry for Primary Industries to perform initial checks.
The snake has been identified by MPI as a Japanese rat snake, which are non-venomous.
It stowed away in a Nissan Leaf which was shipped on March 21 from Nagoya, Japan, and arrived in New Zealand on April 7.
An MPI spokesman said they will be working with the operator to find out how the snake wasn't spotted.
"Snake interceptions are unusual. MPI only intercepts one or two a year.
"They are normally not venomous and mostly arrive dead, due to fumigation."
The 40cm-long reptile emerged from the vehicle at a car yard on Auckland's North Shore yesterday afternoon.
The car is owned by Corporate Cars in Glenfield, and director David Khan said it was being held at the car yard before it received compliance to be registered.
"Maybe it was one of those leaf-eating snakes, and decided the [Nissan] Leaf was one to go for."
The rat snake is not the first to find its way to these shores in unconventional circumstances.
A North American southern ringneck snake was caught in a Mt Maunganui florist shop in 2002.
The male snake, 25cm long and 1cm in diameter, was killed on the spot by a store worker.
A woman who had got "quite close" to the Nissan Leaf said she was surprised to see part of a live snake sticking out from its bonnet.
The woman, who asked not to be named, said she wasn't scared, because the snake seemed to be pinned in there.
"But it was alarming because I don't want snakes moving into the country. It was quite a pretty little snake, waving its head around, but it's alarming anything can come into the country and gets missed.
"This car had a sticker saying it had passed its biosecurity, when clearly it shouldn't have."
Car yard director Graeme McPheat said he was told it wasn't likely to make a bid for freedom.
"The [biosecurity] expert here is saying that because they are in a cold climate, it's very unlikely it would have gone anywhere other than inside the car, where it's warmer. We are 100 per cent confident it's inside the car."
Mr McPheat said the officials were to have the car fumigated.
He said the vehicle was being stored at his yard and was not for sale.
It was awaiting a mechanical compliance check and was stored in an area where the public should not have gone.