Fair Trading Act 1986, ss 37–46; Fair Trading (Infringement Offences) Regulations 2014
You can start by asking the trader to fix the situation. If that doesn’t work, you can:
Take the matter to the Disputes Tribunal, for matters up to $30,000 (see: “The Disputes Tribunal”).
Apply to the High Court for an order to stop the trader breaching the Act (“an injunction”), or for compensation. For applications to the High Court, you’ll usually need help from a lawyer.
Inform the Commerce Commission (see below).