Annah Sophia Stevenson and son Luca were surprised and disgusted to find an insect in their McDonalds meal last month.

A Blenheim woman says she will never eat a Big Mac again after finding a cockroach in her burger.

Annah Sophia Stevenson, 26, had taken three bites of the burger before spotting the large insect.

"I was working on it for about three or four seconds, I thought it was a bit of gristly meat," she said.

DEREK FLYNN/FAIRFAX NZ Annah Sophia Stevenson, with son Luca, 3, chewed on a cockroach after taking a bite from her McDonald's Big Mac burger.

"I pulled it out and it was a bug - half there and half chomped."

She's not certain she ate the other half of the cockroach, but when she pulled the insect out of her mouth, not much of it was left, she said.

"The guts were hanging out and it had a big hole in the middle of it."

McDonald's head of communications Simon Kenny said the incident would be investigated.

"We take any complaint regarding food safety very seriously," he said.

"The franchisee has contacted Annah and is organising to pick up the Big Mac so that they can investigate further."

Stevenson, a cosmetic artist, had taken her son, Luca Curnow, 3, to the McDonald's drive-through as a treat about 5.20pm on Saturday.

She usually avoided fast food, but said it had been a busy day and she hadn't done the groceries. "We're normally the healthiest people, but it was a grab and run thing."

Stevenson ordered a Big Mac Combo, and a McNugget Happy Meal and a cheeseburger for her son, as well as an icecream sundae and an apple pie for dessert.

She waited until she was home to start eating the burger.

Finding the bug had messed with her head, she said.

"I'm the type of person who soaks broccoli in salt water because I'm scared of bugs," she said.

"I was trying to squeeze my teeth through it - I was chewing on that thing for a while."

Not wanting to alarm her son, she spat it out, got up quietly and went to the bathroom.

"I was just retching and vomiting into the sink."

She scrubbed her mouth with a toothbrush, but then found another piece of evidence.

"That's when I got the black crispy shell out of my teeth," she said.

"I just couldn't cope, it felt like every time I swallowed I was swallowing bits of it."

The incident was like something out of Fear Factor, she said.

She had yet to contact McDonald's in Blenheim, but did not want to be compensated with McDonald's food vouchers.

"I'll never, ever eat another Big Mac as long as I live."

While the cockroach in her burger was unlikely to be deliberate, it was a negligent act, she said.

"It's fast-food extreme - they made it so fast they didn't know what they were putting in it."

Despite fears posting something about the incident on Facebook would lump her with the name "Roach Girl", Stevenson posted pictures of her experience on Saturday night.

By Sunday afternoon, 275 people had shared her post.

"I'm still dealing with it," she said.

"I don't do bugs, let alone in my food."