Port workers unpacking an order of ornamental palms in Auckland found an unwanted stowaway when a boa constrictor eyed them from on top of a crate.

The 80cm snake was discovered by port officials yesterday afternoon in a shipping container of plants which had come from Guatemala.

The heavy-bodied snakes are known for their bone-crushing strength and fierce bite, but the Ministry of Agriculture and Forestry said this young boa posed little threat.

"Unless you're a rat or mouse he is not dangerous," said ministry spokesman Stuart Rawnsley.

"But you wouldn't want to leave it alone for too long - they get up to about four metres."

Two MAF quarantine officials captured the snake quickly and easily.

One of them, Brian Robinson, said he was not spooked because snakes and other pests commonly made their way into containers.

"We're used to it, and trained for it. A boa is something different, but it wasn't hard to catch."

The grey-brown boa had a distinctive red-brown "saddle" near its tail section.

It was subdued when handled by officials in the glare of an office yesterday.

The snakes are nocturnal, and more at home in a humid climate with rainforest cover.

The boa will be put down by a vet this morning.

"There's no other option for it," said Mr Rawnsley. He said the boa was not a rare species, so it was not necessary to send it back to Central America.

Under biodiversity laws, snakes were not allowed to be kept in captivity in New Zealand. They were common carriers of disease.

Mr Rawnsley said it was not unusual for snakes to arrive at the ports.

He said any business which handled shipping containers had trained staff who alerted authorities if pests were found.

However, New Zealand borders were not completely watertight, Mr Rawnsley said.

At least 400 pests such as insects, snakes, frogs and spiders made it through border checks each year.

* Found in South America, Central America and some islands in the Caribbean.
* Can grow to 4m in length and 27kg in weight.
* Adult boas are capable of crushing humans and large animals.
* Distinctive colouring makes boas popular among private collectors.
* Boa's natural habitat is rainforest, and it is capable of swimming in rivers and streams.