Labour has promised to ban big companies from charging excessive prices, if elected.

Leader Chris Hipkins said a handful of big businesses were guilty of price gouging - that is, charging more than necessary for essential items.

The full policy includes:

• making it illegal for big companies with substantial market power to charge excessive prices for essential goods and services, or to pay unfairly low prices to suppliers
• setting a legal test for price gouging: holding prices "well above what they would otherwise be in a competitive market, sustained over time"
• requiring companies that gouge to pay penalties and hand back every dollar of excess profit
• giving the Commerce Commission power to investigate price gouging and supporting consumer groups and suppliers to do the same.

Williams said similar rules were already in place in the United Kingdom, Europe, Canada, and Australia.