When Glyn Eason was hitchhiking around the South Island at the age of 18, the idea of swimming across the Cook Strait was planted in his mind.

Now, at 45, the Rotorua man has just achieved the feat, swimming in frigid 15C-17C water for seven hours and 15 minutes wearing only togs, a swimming cap and goggles.

Eason completed the 25.8-kilometre swim from Ohau Bay near Wellington to Perano Head in the Marlborough Sounds on April 16.

He said he felt “relief” and elation when he finished. He put on some warm clothes and gave his wife and daughter a hug.

“And then just had something to eat ... a chicken sandwich, I think,” he said with a laugh.

Eason said his preparation for the swim had been a two-year journey, with him training in the mornings, evenings and on weekends.

He said the “hardest part” was getting used to the cold water and he had been going to Hamurana Springs, where the water was 12C.

“I’ve been going out there four nights a week all summer, just sitting in the water for about 30 minutes ... just to build up a tolerance to the cold water.”

He swam in the Blue Lake throughout winter and also trained with a squad in Rotorua, swimming up to 50km a week since Christmas.

Eason explained the swim could only be done during a neap tide: “That’s the smallest distance between high and low tide, which means there’s less water moving in the strait.”

Glyn Eason took seven hours and 15 minutes to swim across Cook Strait.

A neap tide happened every two weeks, but wind conditions also needed to be favourable.

On the morning of April 15, he found out the conditions were looking good, so he and his family drove to Wellington.

The following day, he met his support crew at Mana Marina and they went to Ohau Bay - the starting point for the swim.

“Basically, they say to you, ‘Swim over and touch that rock’. And when you touch the rock, you put your hand up. They blow a whistle, and then ... you start swimming.”

Eason said a small support boat travelled alongside him, with a navigator and a “feeder”. He had “liquid-based feeds” every 30 minutes.

A bigger boat carrying his wife and daughter travelled with him too, about 50 to 60 metres away.

He said the water was about 17C, with the last hour being about 15C.

Glyn Eason's swim tracked across the Cook Strait. Image / Supplied

About halfway through, Eason said he was feeling “quite fatigued”.

“And then all those dolphins turned up. I thought, ‘These are here as a bit of a sign to give me strength to carry on’.”

Eason said he had prepared for the swim physically but was challenged mentally.

“When you’re halfway through, you naturally feel your doubts ... I just broke it down into 30-minute blocks because you’re feeding every 30 minutes.

“When you’re such a long way from something and you’re looking at it, it just never seems to get any closer.”

Asked if he ever felt like giving up, Eason said: “I was never going to give up.”

“But after three hours I was feeling quite fatigued, and I did have thoughts of, ‘Can I think of a good excuse to get out?’

“But I couldn’t, so I thought, ‘Oh, bugger it, I’ll keep swimming’.”

After the swim, he and his family returned to Wellington for the night.

The night before his swim, he had gone to the beach and said a prayer to Tangaroa (the Māori god of the sea) to give him “safe passage”.

Afterwards, he returned to thank Tangaroa for protecting him.

Eason remembered being 18 and going on the ferry across the Cook Strait.

“I thought, ‘Man, I want to swim this one day’.

“And that’s what planted the seed.”

His next challenges were swimming across the Foveaux Strait or Lake Taupō, saying they were the “triple crown” of swimming in New Zealand.