Boaties should stay well away from Astrolabe Reef and the oil slick from the stranded ship Rena. And if your boat is affected, get advice on cleaning it, according to Maritime NZ.

The heavy fuel oil leaking from the wrecked container ship has spread many miles from Astrolabe and is a major hazard for boats.

A boat gets towed back from the Astrolabe Reef where container ship Rena is leaking oil.

Public relations officer for MNZ, Blair Harkness, says it's really important that boaties stay well away, as the toxic HFO will damage boats if not cleaned correctly and has the potential to further affect the environment if precautions are not taken when cleaning.

Detergent in the water will help shift the gunk if it does get onto boats. It must not be cleaned off where it is likely to wash down into the stormwater system; such as driveways. Hydrocarbons will foul the stormwater system and eventually end up back in the harbour.

For trailer boats, he recommends using a carwash designed for handling such hazards, or washing in an area where the waste can be contained and not fall into stormwater drains.

Larger moored boats and those in marinas will be a lot harder to treat if they are affected by the slick. Most will need to be hauled out at marinas, but travelifts cannot accept boats with oil contamination because this would damage the slings.

The best option is to stay well away from the hazard zone.

Heavy fuel oil on the motor of the Waterline boat, after a journey to Astrolabe Reef.

The Waterline Magazine boat was deployed at the scene throughout the week for SunLive coverage, and passed through an area of oil slick near the 1000m exclusion zone on Friday. The oil slick extends many miles beyond the exclusion zone and shifts, depending on wind and sea conditions.

The pontoon boat emerged coated in a tar-like substance, which smothered every part of the underwater surfaces and stuck in every crevice of the hull and outboard leg.

It took several hours to clean with spray-on degreaser and detergent over a gravel pit.

Fortunately the oil was at the waterline and didn't get into the engine's cooling system.

This would create major problems for any boat, if the sludgy heavy oil was to pass through the intakes.

The HFO is particularly harsh on some hull surfaces and we have reports of it staining fibreglass gel coats and two-pot paints.