The Commerce Commission says it will keep the identities of whistle-blowing grocery suppliers secret if they're worried about reprisals.
The commission is investigating allegations of blackmail and anti-competitive behaviour which have been made against the Countdown supermarket chain by Labour MP Shane Jones under parliamentary privilege.
Mr Jones accused the Countdown supermarket chain of demanding retrospective cash payments from some suppliers to make up for lost profits.
The Northland MP said suppliers are scared to come forward in case they are blacklisted by the chain and has lodged a complaint with the commission.
The commission is asking anyone with information about the allegations to contact them and is offering to keep their identities secret if they wish.
Prime Minister John Key has said he will support an inquiry into the treatment of suppliers by supermarket chains, given the importance of the industry to New Zealanders.
Australia's Progressive Enterprises, which owns the Countdown stores, strongly denied the allegations and invited any investigation into its practices.
In Whangarei, motorists tooted in support of protesters outside a Countdown supermarket on Friday.
Local Labour Party activists led the demonstration in support of Shane Jones. The group of about 10 held placards accusing Countdown of bowling underarm and demanding the Australian company give New Zealanders a fair go.