Coca-Cola has hired a security firm to shadow its Invercargill staff as they make deliveries after death threats were made against them.

Coca-Cola Amatil New Zealand managing director George Adams yesterday confirmed an Invercargill man had threatened to kill Coca-Cola staff working in the city.

Since the threat, staff had been asked to wear plain clothes at work, were travelling in un-branded vehicles, and were accompanied by security officers.

The man said his wife died in February last year and his claims she was poisoned by Coca-Cola were "patently untrue" but his threats against staff in Invercargill were being taken serious-ly by the company, Mr Adams said.

The man was "very irate".

Coca-Cola received a letter from Otago-Southland coroner David Crerar about a month ago, asking for comment on an inquest for the woman.

She was believed to have consumed up to 10 litres of soft drink a day and eaten very little, and died early last year, Mr Adams said. He had seen just a paragraph from of the report, which suggested the woman had been deficient in minerals such as potassium.

He was unsure how long the company would continue with its higher level of security for staff.

"Really, until the police tell us they don't believe there's a threat," he said.

The company had about 20 staff in Southland.

Senior Sergeant Bruce Terry, of Invercargill, said police were aware of the matter and an officer had spoken to the person involved.

He declined to comment further.