A healthcare worker has been left "disgusted" after finding a maggot in her McDonald's burger box.

Linda MacDonald, a mental health carer, had just finished eating an Angus Burger Combo, which she bought from the Pt Chevalier McDonald's, when a colleague she shared the burger with pointed out something "wiggling" in the box.

The 59-year-old Auckland grandmother spat out her remaining mouthful and ran to the toilet to throw up.

"It was awful," she said. "They offered me McDonald's vouchers, and I told them: 'No way am I ever going to set foot in there again'. The cheek of it - it's so wrong."

McDonald's NZ boss Mark Hawthorne said he did not believe the maggot came from within the restaurant. It was dead when the company conducted tests.

But Mrs MacDonald said the maggot was alive when she found it and it was not tested until six days later.

"They more or less accused me of putting it in there myself. How dare they do that? I mean, no way. I'm a responsible person. To be accused of something like this really annoys me ... it really hurts."

Mrs MacDonald, who ate her lunch with several colleagues, said she didn't want anything from the company except an apology.

She says she cut the burger with a knife when she shared it with her colleague, but is adamant the maggot did not get there then.

While acknowledging there will never be proof either way, she remains upset about the company's reaction.

After rejecting the McDonald's vouchers, Mrs MacDonald was sent $100 of Westfield shopping mall vouchers, which she gave to her son.

Her colleagues at Buchanan Rehabilitation Centre who saw the maggot have also been put off, and several say they won't buy McDonald's food again.

Mr Hawthorne said the company had conducted a thorough investigation, which included the restaurant, suppliers of ingredients and independent laboratory testing.

"The larva returned to us was dead, not exposed to heat, and was not associated with the Angus beef patty. It is hard to determine the point of contamination, however, due to the circumstances around the purchase, and in light of the expert advice received and thorough checks completed, we believe it is unlikely to have resulted from McDonald's.

"At all times during the process we have been empathetic to Ms MacDonald's concerns, and acted in good faith to keep her informed through the investigation."

The owner of Auckland business Real Nutrition, Jacquie Dale, said the presence of maggots usually indicated food had not been stored correctly or was past its use-by date.

"If there's a maggot in your meal and it didn't fall from the sky into food then it's pretty safe to say a fly or its entire family have used your food as a nesting ground."

A customer at McDonald's in Whangarei last year claimed to have opened a Big Mac burger and found maggots crawling around on top of the patty.

A health authority investigation was inconclusive because of storage conditions and the delay before the burger could be tested.