A group of friends have started an epic journey down the Waikato river this morning to raise money for a Bay of Plenty boy with a rare form of bone cancer.

The four friends from the Omanu surf life-saving club in Mount Maunganui have already raised around $35,000 for 10-year-old Harrison Mundy and hope to raise more for the Papamoa boy by the time they finish their stand-up paddle boarding journey from Lake Taupo to the Port of Waikato.

The group set out just after 6.20am and plan to paddle up 80km a day to finish the trip in around four days, support person Jimi Grant said.

"There will be some tired bodies, it's quite an exercise doing 60km to 80km a day," he said.

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As well as the paddling, the group will have to get around some dams on the way and will be camping out at night.

Mr Grant hoped they could raise another $5000 to $10,000 along the route.

Harrison has been diagnosed with Ewing Sarcoma - a rare form of cancerous tumour that grows in bones or the soft tissue around them.

Ewing Sarcoma makes up only 1 per cent of all cancers diagnosed and typically affects children.