An estimated 2000 boat hulls will be checked by divers between now and mid-May for a range of unwanted marine pests that are threatening Northland's marine environment.

Northland Regional Council biosecurity manager — marine and strategy, Sophia Clark, said Northland-based dive contractors Marine Environmental Field Services officially began the 2018-19 inspection programme in Whangārei Harbour last month.

"We've been carrying out these hull inspections for several years, and are especially keen to ensure vessels are free of marine pests before their owners start travelling to new areas with the onset of warmer summer weather," she said.

Two thousand hulls, ranging from recreational craft and fishing boats to superyachts and barges, had been checked over the last inspection season, and with pests like the Mediterranean fanworm now established in Whangārei Harbour, and recently being found at Ōpua, the owners of vessels travelling from those areas to other locations were being urged to be extra vigilant.

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Ms Clark said as well as rules covering the transportation of marine pests, the council now set limits regarding the amount of biofouling (build-ups of micro-organisms, algae, plants and pests) vessels were permitted to carry.

The plan demanded that any vessel entering Northland waters, or moving from one harbour to another, must have no more than 'light fouling,' defined as a slime layer and/or barnacles and a few patches (up to 100mm in diameter) of macrofouling.

The owners of vessels found with more than light fouling would be notified so they could ensure it was clean before moving to a new area. If a pest species was detected, owners would be asked to haul the vessels out and clean or treat them, a potentially costly process. A $500 fine may also apply.

Ms Clark said the council was very appreciative of the steps many owners were already taking, and had been impressed by a noticeable change in the attitude to boat cleanliness in recent years.

"As an example, of the 140 vessels inspected over the last Christmas period, just two had a marine pest species present." Those pests had been concealed in 'niche' areas, which required special attention when applying anti-fouling or 'lifting and washing'.

"Council strongly suggests double-checking these niche areas, like the base of the keel, behind anodes and bow thrusters before your vessel goes back in the water."

Northland's marina operators had also been very supportive.

"Marina operators have already been requiring a receipt proving a vessel conforms to what's been dubbed the 'six or one' initiative, either being anti-fouled within six months of leaving a known fanworm-infected area, or having undergone a 'lift and wash' within one month of leaving an infected area like Auckland," she said.

■Details of the council's rules and requirements can be found at www.nrc.govt.nz/marinebiosecurity