Trampers in the Abel Tasman National Park wade through water after a bridge near Marahau was washed out by Cyclone Fehi. The bridge has since been replaced.

Part of the Abel Tasman coastal track has fallen into the sea, a campsite has been closed and other areas of the park have been damaged in the wake of two major storms.

The Department of Conservation is still assessing the damage from ex-cyclones Fehi and Gita, that struck within three weeks of each other in February. It faces a repair bill estimated to be in the hundreds of thousands of dollars.

Department of Conservation Golden Bay senior recreation ranger Neil Murray said staff thought they had escaped the impact of Fehi in the north of the park, but about 10 days after the cyclone on a sunny afternoon, a section of the track north of Totaranui fell into the sea.

Department of Conservation Sand covering the Bark Bay campsite after Cyclone Fehi in the Abel Tasman National Park.

"Even some people swimming in the ocean down there saw it happen, which would have been pretty remarkable."

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Analysis by a geotechnical engineer found the storm surge during Fehi had eroded the bottom of a steep granite slope which caused the slip between Anapai Bay and Mutton Cove.

Department of Conservation Trampers on a bridge at Anchorage in the Abel Tasman National Park that was damaged during Cyclone Fehi.

The track was now at Anapai Bay and visitors travelling from Totaranui north, now had to do so via the steeper Gibbs Hill Track which "certainly added a bit more effort to the walk".

"There is no immediate detour available...now we are going to have to look for a re-route, which is going to take us some time."

Over the last two weeks, DOC staff had been scouting for a new line for the track. Murray said it could take up to 700m of new track to get around the slip affected area, which would then take a few months to build.

BRADEN FASTIER/STUFF Trampers walk across the estuary at Torrent Bay in the Abel Tasman National Park, before Cyclone Fehi hit in February.

As an interim measure, water taxis had been approved to drop people at Mutton Cove, bypassing the slip.

Department of Conservation Motueka operations manager Mark Townsend said major coastal erosion had occurred during Fehi and several campsites were inundated with sand and sea water. A 50 metre bridge near the entrance to the track near Marahau was pushed sideways by the tidal surge.

Then two weeks later when Gita hit, another 15 metre bridge near Marahau was destroyed. A temporary boardwalk had been erected in the interim and and an engineer had visited the site, with a plan for the bridge to be replaced in April.

"The section of track between Marahau and Onetahuti has survived both storms really well," Townsend said.

"It was a miracle we didn't actually get more track damage."

Townsend said the decision had been made to close the Tonga Quarry Campsite due to erosion. The storm surge during Fehi had been so strong it had shifted blocks of granite on the beach.

The Bark Bay campsite had half a metre of sand over it and a lot of the trees on the sandspit were dying from salt water intrusion. The Mosquito Bay campsite also had half a metre of sand across it and Townsend said it was likely that site needed to be shifted to higher ground.

He said planning for future events meant the "gradual retreat" of campsites at some locations.

Townsend said DOC was still finalising the costs associated with the damage, but it was likely to cost several hundreds of thousands of dollars. About $30,000 had been refunded to visitors in camp and hut fees who had been unable to access the park in February.

More restoration work was planned for the dunes at Anchorage to protect it from future coastal inundation.

Over $2 million was invested in infrastructure at Anchorage in 2013, building a new hut, campsite and sewage facilities. Between the hut and the campsite it had the capacity for 134 people.

"We are never going to hold back the force of the water and Mother Nature, however, we can work with it."

It wasn't all bad news, Project Janszoon restoration planting supervisor Helen Lindsay said while new plantings in the park had been washed away with the coastal surge, it was a positive that salt water inundation had also killed gorse in some areas.

The two beaches where plants had been washed out were Anchorage and Medlands.

The dunes at Anchorage had been restored as part of an initiative between Project Janszoon and Motueka High School students with a focus on removing gorse to reduce the fire risk in the park.

Native tussock, such as spinifex, had been planted to stabilise the dunes and provide ground cover. Lindsay said it was a resilient plant, that could cope with being buried in sand and silt and was able to re-establish itself after storm events.

"If we do get a washout of the plantings that they have done, it is not a disaster, that is just what happens."

The impact of the storm surge from Fehi and the high tide occurring at the same time had not only washed out new plantings, but also trees that were more than 50 years old, which was "pretty unusual".

While it was hard to know how frequent such storm events would be, Lindsay said there was likely to be periods of stability along the shoreline which would give plantings time to recover.

"If we have got the right species, and enough of a population of the right species then they will gradually re-establish themselves."