There are claims a number of regional KFC restaurants are so understaffed customers are abusing already-under-pressure workers.
Over the last six months some stores, including Blenheim and Waihi, have been chronically understaffed, Unite Union says.
Frustrated customers are becoming increasingly angry and taking it out on staff, National Secretary Gerard Hehir says.
Staff have reported having things thrown at them, he says, and when customers are trapped in the drive-through for more than half an hour it's inevitable they'll get angry.
Staff numbers were slowly being resolved, Mr Hehir said. However, it was still tough on those struggling to keep the businesses running.
Unite was encouraging staff to close the drive-through or the whole store if they only had one person rostered on, he said, which had happened on some occasions.