Two adventurers who have paddled 3,000 kilometres (1,864 miles) through 10-metre swells and powerful currents in an attempt to cross the Tasman Sea are on the verge of giving up only 170 kilometres from the finish.

James Castrission, 25, and Justin Jones, 24, are so physically and mentally exhausted that they are losing their sense of direction as they approach New Zealand.

Persistent headwinds and strong ocean currents have taken their toll and forced the Australians kayakers to paddle 800 kilometres farther than planned and prolonged their voyage by two weeks.

Concerns are now growing about the health of the pair, whose leg muscles are wasting and whose fat reserves are thought to be entirely depleted. Mr Jones said in a podcast that he and his partner, who set out on November 13, were losing mental focus.

“We'll get our east and west mixed up and can't remember numbers we were talking about just an hour ago,” he said. “Normally, we're really switched on guys [but] we're just getting worn down slowly. We're busting out 30, 40, 50km a day and at the moment it's just bleak weather. The sea's grey and it's rolling in and blending into the sky, which is grey. Everything's grey.”

They have one freeze-dried meal a day and since their electric desalination pump broke last month they have been forced to use a manual version to produce drinking water.

Tom Mitchell, a spokesman, said: “Their bodies have started to deteriorate, their leg muscles are definitely wasting. They're chewing into their fat stores.”

In the back of their minds is the knowledge that Andrew McAuley, a solo Australian kayaker, was only 50 kilometres from New Zealand last February when he sent a garbled distress call. Emergency crews recovered his upturned kayak but his body was never found.

If the two adventurers reach New Plymouth, on the west coast of the North Island, by Saturday, they will become the first to cross the Tasman Sea in a double kayak. They were defeated, however, in their quest to become the first Australians to make the crossing by a foursome who completed the journey in the opposite direction in 31 days on December 30. The first person to paddle the Tasman was Colin Quincy, a New Zealander, in a solo kayak in 1976.

Taking on the Tasman Sea, known as “the Ditch”, is considered extremely perilous. The Roaring Forties, the name given to the latitudes between 40 and 50 degrees south because of the boisterous and prevailing westerly winds, produce some of the stormiest seas in the world. Wave heights regularly exceed four metres, compared with two metres in the subtropics, and in the milder summer months winds still reach more than 35 knots.

But Mr Jones said they were determined to finish their journey. “We're just going to keep focus on what we do, double-check everything and just keep going ... just keep going.”

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