Fifteen-year-old Lachie Eastwood has completed a surfing marathon, hitting the waves every day for 71 days in a row, to honour his grandfather Steve Eastwood.

His beloved Poppy passed away in November after being diagnosed with cancer in 2018.

Lachie started his surfing mission soon after to raise money for the oncology unit at the new Phillip Island Community Hospital, due to open at the end of the year.

He hit the 71-day milestone on February 3 and within days his extended family and friends have already raised almost $600 for the cause.

His mum Carrie said it was an amazing achievement.

"The final week was a struggle, as he was juggling school, work, footy training and then fitting in a surf," Carrie said of the marathon effort.

"He went out rain, hail or shine. Not all days were ideal, but he persevered.

"He was determined to do it for his Poppy, even when the surf wasn't great.

"He had early mornings and late nights and surfed solo on many of the days."

She said anyone wanting to support Lachie's effort or contribute to the fundraiser for the new oncology unit could deposit money into a community account at the Bendigo Bank.

"All money from the account will be handed over when the Community Hospital opens," she said.

Donations can be made online or in person at a local Bendigo Bank branch.

Account name: Oncology
BSB: 633-000
Account: 198858409

Family members are writing Lachie as a reference for their donations, so the bank can tally how much he raised, and anyone wanting to support Lachie's incredible 71-day effort can do the same.

Back in 2022, Lachie surfed for 100 days straight, just as a personal challenge.

At the time, he told the Advertiser he did it because "I love surfing and I just wanted to surf more", and said he'd look at similar challenges - but perhaps doing it for a cause.

True to his word, Lachie has done just that and made an incredible contribution to future oncology services on Phillip Island.