After spending two days running, cycling and kayaking across the South Island, Nikki Kaye's body is "completely munted".

The Auckland Central National MP, who had a double mastectomy after breast cancer in 2016, used every ounce of energy she had to finish the gruelling Coast to Coast – a 243km race from Kumara's Serpentine Beach to Christchurch's New Brighton Beach.

She finished in a time of 18 hours and 54 minutes.

ROSS GIBLIN/STUFF Kaye was diagnosed with breast cancer in 2016, but has continued to challenge herself physically.

"I was very much at the end of the pack, but I've always been at the end of the pack for the Coast to Coast. I'm just so happy to complete it."

Kaye, 39, completed the event in 2008 and 2013, and last year did the run section, but this was her first time in the two-day event since breast cancer.

JOSEPH JOHNSON/STUFF The 2020 running of the Coast to Coast multisport event.

She even managed to shave 30 minutes off her 2013 time and close to 1.5 hours off her 2008 time.

"Day one I found really hard."

Before the start of day two, she almost gave up.

"It was cold and I was just so tired. I had no energy. I got in the boot in a sleeping bag and fell asleep for half an hour."

At one point one of her support crew joked "shall we just drive straight toward Christchurch", but Kaye knew she could not give up.

"I just tried not to think of the whole race. I thought 'I'll get my shoes on and get to the start, then I'd do 5km more'."

She had also convinced a couple of friends to do the race and felt like she had to keep going.

"I couldn't convince a couple of mates, then not turn up."

The 70km kayak along the Waimakariri River was the most challenging section for her, especially after having a mastectomy.

"I just didn't have any confidence that I had any strength. I felt like I was building from ground zero."

She described the conditions as terrible. It was wet and windy and the river was fast running.

"I was just trying to hold on to the paddle and stay up right, let alone actually power through. I was airborne in bits of it. I think because I am quite light and the wind was just so high I spent 20 minutes just trying to stay up right."

At one point, former Christchurch City councillor Glenn Livingstone paddled alongside Kaye and gave her a chocolate chip muffin. The two are on completely different sides of the political spectrum, but Livingstone's generosity was a great example of the spirit of the race and camaraderie shown by competitors, Kaye said.

"The thing about the Coast to Coast is so many people that do it have extraordinary stories. The number of times that people would ride past me and say 'are you all right, jump on'. It's probably one of the most amazing things about the event."