Fast food giant McDonald's shut down up to 50 stores across the North Island on Friday night after an incident at a plant meant there could have been glass in burger buns.

About half of its 100 North Island stores were closed between 10.30pm on Friday and 3am yesterday as potentially injury-causing stock was removed.

McDonald's spokesman Simon Kenny said the company was informed on Friday night that broken glass from a light was found on the floor next to the production line and the suppliers could not guarantee glass had not entered the buns. Suppliers told McDonald's quality assurance staff: "We see it as extremely low risk but, no, we can't guarantee it."

Kenny said the last time maintenance was done was last Sunday so all buns made between Sunday and Thursday had to be pulled from the stores. Only North Island stores were affected as they got fresh buns - South Island stores were not affected as they received frozen buns made before the glass was found.

General manager of North's Bun Company, which makes the chain's buns, referred all questions back to McDonald's.

McDonald's took all of Saturday to finally reveal the nature of a problem that had angered customers, with many going to McDonald's Facebook page to complain about not being able to get their burger fix.

The company's public relations machine reassured customers that the incident was minor, in one post saying: "One of the plants we use for our buns experienced a breakdown and we ran into a bit of a bun supply issue late last night".

Auckland's Great North Rd McDonald's store manager Julia Maa said the restaurant closed between 10.30pm and 2am, which resulted in "a lot of angry drunk customers".

She said head office had informed them it was a problem with the buns, but did not go into detail. A big morning delivery ensured the store's breakfast was not affected.

Auckland medical officer of health Richard Hoskins said he had not been contacted, but would have expected to be if there was a "risk to human health" or if there had been a formal food recall issue through the Ministry of Primary Industries.

Among those posting on the company's Facebook page, one customer said he had been told the buns were "contaminated" - an allegation rejected by McDonald's, which posted a response saying the issue was caused by a "breakdown".

Auckland man Aaron Levi said he went to the Constellation Drive McDonald's about 11pm. The store was closed and an employee told him "all the buns had to be sent back due to contamination issues".

Levi, who took his concerns to Facebook, was told online: "They have not given you the correct information. There was a bun supply issue and restaurants are being resupplied through today."

Kenny denied McDonald's had misled people on Facebook. He said McDonald's told those customers they had been given "incorrect information" and it was a "bun supply issue" because there was a "very low risk" and it was highly unlikely glass was in the buns.

The company was compiling reports to take to authorities on Monday, he said.

"One thing we know is there's been absolutely no reports from the public [of any injury]."

- © Fairfax NZ News