The Commerce Commission has confirmed it has issued notices legally requiring people to assist in its inquiry into the Countdown supermarket chain.
Accusations of anti-competitive behaviour are being investigated.
Under section 98 of the Commerce Act, the commission can issue notices to supply information and documents, which is common practice.
Labour MP Clayton Cosgrove said that will help ensure all relevant information is included in the inquiry.
He said the commission needs to get as much evidence as possible to complete a full and robust inquiry.