Nine cane toads have been discovered in a truck-load of mango plants sent to Perth, in what is believed to be the large case of the toxic pests hitch-hiking south.

The toads were accidently packed into the consignment in the Kimberley town of Kununurra, which has been over-run with toads for more than a year.

By the time the truck travelled more than 2,000 kilometres to Perth, two of the toads were dead but the others were still alive.

Wildlife officer Teagan Johnston said the toads were not found until the truck was opened at Toll Express depot in Perth.

"We applaud the trucking company for doing all the right things," she said.

"They closed the truck up, we went down, and in opening and investigating we found nine cane toads."

She said the toads could have had a fairly pleasant journey.

"We weren't surprised some of them were still alive," Ms Johnston said.

"The truck obviously had fresh produce in the back, and some mango plants, and so it was probably a nice environment for them.

"I suppose more concerning for us was the number of cane toads that were found."

Ms Johnston said that if the toads had escaped and sourced water, they could have started breeding in the Perth area.

"It brings home the message that it's a shared responsibility, and we've got to be vigilant about making sure we are checking consignments coming down from the north," she said.

"It's not just commercial consignments that pose a risk, but even in your own car, caravan or luggage.

"It's possible for them to travel as they're out there and it can happen so it's important we work together to stop them."

The Department of Parks and Wildlife recently recruited a new cane toad sniffer dog in Kununurra, but it is not able to check every truck that leaves the area.

Department staff will be contacting the company that packed and sent the mango plants, to ensure it has biosecurity plans in place.

The toads will be euthanased later this week.