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A McDonald's restaurant has apologised to a mother whose son found maggots crawling through his Big Mac - and given her $135 of vouchers for more burgers.

Northland health authorities are investigating a complaint from Lianne Tansley about the burger sold at Whangarei's Bank St restaurant.

Her 15-year-old son Isaac has vowed to never eat McDonald's again - but the fastfood giant says its food safety procedures are stringent and it is keen to investigate what happened.

Isaac ordered a Big Mac and cheeseburger from the drive-through on New Year's Day. He finished the cheeseburger but didn't start eating the Big Mac until he and his mother were nearly at their Whangarei Heads home.

"He took the top bun off to take out the gherkin, and then he said, 'My God, Mum, look at this'," said Lianne Tansley. "The whole patty was moving as if it was alive. It was gross." When she rang McDonald's a manager took her name and contact details and asked her to bring the burger back in to be replaced. "I said, 'No thanks, I'm never eating there again'."

Nine days later, after Tansley sent a photograph of the burger to the Northland District Health Board and the Northern Advocate newspaper, McDonald's regional operations manager Sanjay Kumar rang her.

He apologised for the delay and said the matter hadn't been brought to his attention, she said.

Kumar told the Herald on Sunday he visited Tansley on Friday and offered the vouchers as an apology.

The complaint had not been followed up promptly because Tansley didn't return the Big Mac to be checked, Kumar said.

"We thought the matter had been dropped. Otherwise we thought it could have been a hoax."

During his visit Kumar took the burger, which will be sent to public health officials tomorrow for testing.

Northland DHB public health leader Paul Reid had seen a photo of the burger and said two officers visited the Bank St premises on Friday. He described the complaint as "routine" but had yet to see a written report - or the burger.

McDonald's national communications manager Kate Porter said it was unlikely maggots could have hatched in the beef patties, which were cooked from frozen when orders were placed.

Strict hygiene rules were in place at McDonald's and the chain took complaints seriously.

Porter urged customers with complaints to return the product so the restaurant could investigate.

"Sometimes things go wrong and we do apologise for that."