Fire investigators were still analysing the scene at noon today, but said the cause was not yet clear.
Riversdale Beach Volunteer Rural Fire Force had been unable to bring the blaze under control.
"The roof had collapsed and we couldn't do anything," fire chief Glen Meredith said.
"The gas bottles were going off and the powerlines were on the ground."
Mr Meredith said the force had modest resources and had to hold tight until back-up arrived.
Four fire engines and a water tanker from Masterton and Carterton drove 45 minutes to the coastal township to help battle the blaze.
Masterton fire station officer Mike Cornford said on arrival the building had "totally collapsed" to the ground.
"It was all just a big pile of burning materials . . . there was nothing we could do except extinguish it.
"The whole structure was burnt out and the roof was on the floor."
By the time Riversdale Beach Holiday Park owners Dion Moss and Michelle Naysmith discovered the blaze, the marae was "too far gone".
"There was a bit of a glow out the window," said Ms Naysmith, who had got up in the night.
"Everything looked a bit more illuminated than usual. After hearing pops and bangs we knew something wasn't right."
Mr Moss went outside to find "smoke wafting" over the park.
The couple called the fire brigade and found the marae "well ablaze".
"We just checked that there were no cars or anybody there, although if there had been it would have been too late."
Trustees of the marae declined to comment.
Photo / Hayley Gastmeier 