The Commerce Commission enforces the Fair Trading Act, which protects consumers against unfair trading practices. Section 12A of the Act prohibits unsubstantiated representations, so if a company is making an advertising claim that they can’t back up with evidence they may be in breach of this legislation.
The Fair Trading Act also covers other aspects of trading, and the Commerce Commission has a number of Fair Trading Act fact sheets for the various topics covered by the act, such as unsubstantiated representations and health and nutrition claims.
Because the Commerce Commission has limited resources, they may not be able to act on every complaint they receive. They have a number of enforcement criteria which they use to determine how to prioritise a complaint, which includes things such as the public interest in the issue, and the amount of harm it could do if left unchecked.
In some cases, with small traders, the Fair Trading Act may not apply. For example, if someone is selling personal items on Trade Me but not running a business, they are typically not considered to be in trade under the law.
You can lay a complaint with the Commerce Commission through their online complaint form.