More than 30 mobile traders - commonly known as "truck shops" - do not comply with the law, according to the Commerce Commission.
The commission has released a report on its year-long investigation into consumers buying goods at exorbitant prices from truck shops.
It found 31 traders did not comply with all of their obligations under the Fair Trading Act, and the Credit Contracts and Consumer Finance Act.
Commissioner Anna Rawlings said the project found mobile trader sales agreements or credit contracts were "generally deficient" and often failed to outline basic details such as what the goods were, the exact payment terms, the total amount owed and the end date.
Two traders were being further investigated and the others have been advised of changes they must make to comply with the law.