To his surprise, the assistant told Mr Haumaha two women had paid for it. The assistant said the women had commented that he had been "blessed by God today".
Mr Haumaha said he was blown away.
"I'm just really stunned, grateful and appreciative ...
"I ran out and tried to find them to thank them."
Mr Haumaha spent the weekend working on the gravestone for a hura kohatu (dedication ceremony) to be carried out in the near future.
"With the wet weather we've had the tarpaulin has come in handy. We didn't finish the work on Mum's grave so we are using it to keep it dry."
Mr Haumaha decided to tell The Daily Post as a way of giving thanks to his mystery benefactors.
There have been other random acts of kindness in Rotorua this year to further prove the city's residents are a kind-hearted lot.
This year The Daily Post reported a "total stranger" sitting in the public gallery of the Rotorua District Court paid for the accommodation of a Slovenian man charged with not paying his bill.
When he appeared in court he was told the charge would be dismissed if he paid the $87 bill, but he said he had no money.
A member of the public sitting in court offered to pay it for him.
Shortly before that The Daily Post reported how a Rotorua pensioner was humbled by the actions of a stranger who paid for his groceries when he found he was a bit short at the checkout.
Tony Grabrovaz found the items he needed came to more than he had in the bank and as he stepped to the side to sort what to leave behind, the next customer in the queue gave the shop assistant $20 to pay his bill.