There are claims a number of regional KFC restaurants are so understaffed customers are abusing already under-pressure workers.

The Unite Union says over the last six months some stores in areas including Blenheim and Waihi have been chronically under-staffed.

National Secretary Gerard Hehir said customers are becoming increasingly angry and taking it out on staff.

"They can be abusive, people have had stuff thrown at them, at drive-through waiting times are taking up to 30 minutes, and people are literally trapped in the drive-through."

"It's almost inevitable that people are going to be angry and irritable."

Hehir said the union is encouraging stores to partially close if staff numbers are too low - for example, shutting the drive-through.

"If there's only one person there, they shouldn't be trying to run when we've had situations like that."