A heavy infestation of mature Mediterranean fanworm on a boat that recently moved around the Bay of Islands moorings has alarmed biosecurity agents.
It has also raised the ire of local boaties who say their boats are getting infected by an invasion of pests already well settled in the area.
A weighty fleece of sheaths, weeds and worms was found on the vessel that was slipped at Opua after having been anchored at Tapu Point and Matauwhi Bay, near Russell.
During the Northland Regional Council's (NRC) 2015-2016 survey season, six vessels were recorded with fanworm in Matauwhi Bay.
"We did a follow up survey of Matauwhi Bay in late July this year which checked the reef, structures, moorings and adjacent hardstand area where incidents had been recorded, but no fanworm was found," NRC biosecurity manager Don McKenzie said.
The boat in question this week was inspected in the same routine biosecurity monitoring programme, with some hull fouling found on it, but no marine pests.
As well as other unspecified action, an instant fine of $500 could be slapped on owners or those in charge of a boat carrying the highly invasive pest, although NRC staff are still considering whether to do so in this case.
Mr McKenzie said officials were alerted to the infestation after the vessel was pulled out of the water at Opua.
"We're also asking boaties - both locals and visitors alike - to be especially vigilant for fanworm and other unwanted marine pests."
That warning went to the nub of the matter as locals may not be able to prevent their boat hulls picking up unwanted pests already established in an area, Russell yacht hire owner Stephen Western said.
It was unfair to fine boat owners whose vessels had picked up the pest "at home", he said.
"It's like having a pohutukawa tree planted outside your house and being fined because it's got myrtle rust," Mr Western said.
He believed the explosion of fanworm, and even the increase in size and numbers of barnacles, was due in large part to the water getting warmer in recent years.
Anti-fouling products for smaller vessels were proving ineffective at repelling hitchhikers, but environmental laws prevented the use of heavier duty deterrents such as those used on large, ocean going ships, Mr Western said.
"The worm is here. The boats are getting anti-fouled and being inspected, and they're still picking it up."
The latest discovery comes as NRC contractors prepare for the annual Northland-wide hull inspection programme.
More than 2000 hulls of all kinds, from small runabouts to luxury super yachts, are expected to be checked over the next few months.
Contractors will survey the mooring area and seafloor where the vessel had been and would also start this year's hull inspection programme there.