Some boat owners might need to change the antifouling they use, according to Northland Regional Council after a hull inspection programme.
It appears that using the wrong kind of antifouling has failed to keep unwanted marine pests from hitchhiking on hulls.
The 2016-17 inspection programme kicked off in Whangarei Harbour recently.
In the first week, 200 vessels moored within the harbour were checked, with 33 - or 16.5 per cent - found to be carrying pests.
The majority were hosting Mediterranean fanworm and were within Parua Bay, where fanworm is known to be established; however, three were in McLeod Bay and one at One Tree Pt.
Another vessel at One Tree Pt had come from Nelson with mature Japanese kelp (undaria) on its hull.
Regional council biosecurity officer Irene Middleton said the boat owners had generally responded positively, with some voluntarily booking haul-outs.
The council will issue notices of direction, requiring owners to come up with an acceptable plan to clean their vessels.
Ms Middleton said the results were broadly in line with expectations.
There were indications that some boat owners needed to renew their antifoul paint more frequently, and to choose the right paint type for their boats.
"Ablative or semi-ablative antifouls are designed to be self-cleaning when the boat is moved," Ms Middleton said.
"However, they're only effective when boats are being moved regularly.
"If owners don't take their boats out as much as they originally anticipated, they would be better off with other types of antifoul."
This might partly explain the feedback from some boat owners that antifouls were becoming less effective over time, she said.
Northland-based dive contractors Marine Environmental Field Services are scheduled to check the underwater surfaces of about 1500 boat hulls between now and mid-May.
Inspections move to the Bay of Islands this week.
Boat owners were welcome to talk to the dive team as they worked their way around the region in a red inflatable, Ms Middleton said.
Alternatively, they could direct queries to the council's Marine Biosecurity team on 0800 002 004 or firstname.lastname@example.org