Boats are welcome in Northland this summer but dirty bottoms are not.
Anyone either towing a vessel into the region by road or coming in by sea is being asked to make sure they leave unwanted marine pests at home.
Joe Carr, chairman of the Northland Regional Council's environmental management committee, is making the plea to visiting boaties to make sure their hulls are clean.
This is especially for those boats coming from Auckland, which has well-established populations of Mediterranean fanworm in its marinas.
Mr Carr said domestic and foreign visitors were a significant contributor to Northland's economy, with many thousands enjoying the region's fantastic environment annually - in particular its 3000km-plus of coastal areas.
About 2000 domestic and international recreational vessels visit Northland annually.
Mr Carr urges everyone to do their bit to ensure those special spots are enjoyed as sustainably as possible, including preventing the spread of pests.
"Fanworm and other pest species compete with native and farmed shellfish, fouling marinas and other structures and Northland is one of only two New Zealand regions to recognise the value of its marine environment with specific rules to prevent the establishment and spread of marine pests," he said.
A summer programme of checking boat hulls for Mediterranean fanworm and other marine pests is already under way.
Last summer, council staff checked about 250 boat hulls, with fanworm found in three cases.
This year, the council is also checking locally-based boats on moorings and in marinas for fouling species that Northland might be exporting to other regions.
If boats are found to carry marine pests, owners must have the hull cleaned at their own cost and as soon as possible.
That usually requires a haul-out as there are restrictions on cleaning boat hulls while in the water.
Mr Carr said special attention must be given to "niche areas" of hulls where marine pests could be missed during anti-fouling.