The adult toad was discovered over the weekend by the nursery owners, who managed to capture it.

They alerted Department of Environment and Conservation officers who then humanely the toad.

Regional wildlife officer Brett Lewis said it was difficult to know how the cane toad arrived in Karratha - but they though a pallet of plants from Darwin could have been responsible.

"We are investigating the possibility that the toad may have hitched a ride on a pallet of plants delivered to the nursery from Darwin," Mr Lewis said.

"All plants delivered to the nursery from Darwin are inspected and sprayed for plant pests and a certificate is issued declaring they are free from infestation.

"There are also vehicle checks in place at the border, but the toads are not always easily found."

The DEC praised the nursery owners for capturing the toad and contacting them.

It urged anyone travelling from the Northern Territory to be on the lookout for toads hitching a ride in freight or under the body of vehicles.

"As cane toads move closer to the Western Australian border, the chances of them hitching lifts on vehicles or freight are likely to increase," Mr Lewis said.

"Until now, all suspected cane toad sightings in the Pilbara have been misidentification of native species of frogs, however, every report received by DEC is taken seriously and investigated."

Anyone who suspects they have found a cane toad should isolate the animal and report the sighting to DEC immediately on 1800 084 881 or 9182 2000.