Expert explains biofoul that halted cruise

The build-up of biological material known as biofoul - which kept thousands of cruise ship travellers unable to enter Australia for several days - could decimate the environment without intervention, an expert says.

The Viking Orion has returned to its scheduled itinerary and is on course for Sydney after docking in Melbourne on Monday.

Biosecurity concerns forced the 227-metre cruise ship to undergo cleaning of its hull before it was allowed to enter Australian waters after travelling from New Zealand.

Organic build-up on ship hulls including bacteria, plankton, sea grasses, mussels and barnacles can enable "stowaway" species to reach Australia, Associate Professor Sophie Leterme said.

The director of the Flinders University Biofilm Training Centre said as on land, the introduction of invasive species to Australia's marine environments was of huge concern.

"If they make it to our reefs, they might decimate the environment and cause some serious issues," Assoc Prof Leterme told AAP.

"You don't want to find a Japanese or French seaweed, for example, taking over the biodiversity of Australian ecosystems."

Particularly on larger ships, cleaning involved considerable expense, required divers and often took several days.

"With COVID, maybe the companies have not kept up with the requirements as much as they should," Assoc Prof Leterme said. 

Australia and New Zealand have relatively stringent biofouling requirements compared to other countries.

The federal agriculture department recently introduced new rules for managing biofoul on international vessels arriving in Australia.

This included tighter requirements for vessels to report on how biofouling was being managed prior to arriving in Australian waters.

While it was not common for cruise ships to encounter issues with not being able to dock due to biofoul, Assoc Prof Leterme said she suspected it was for tankers and other vessels.

"I guess it just doesn't make the news because there's not that many people involved," she said.

The agriculture department said in a statement it was notified of "small amounts of biofoul" on the hull of the Viking Orion on December 28.

Divers cleaned the hull while the ship was anchored outside Australian waters, approximately 22 kilometres offshore from the port of Adelaide, according to the department.

A Viking spokeswoman said the company was working with passengers regarding compensation for the impact on their voyage.

"Following the exterior cleaning of a limited amount of standard marine growth (commonly known as marine biofouling or algae) from the ship' s hull - a routine cleaning procedure for nautical vessels - the ship unfortunately missed several ports on this itinerary," a Viking spokeswoman said.

Viking Orion is due to arrive in Sydney on Wednesday.