The Commerce Commission is warning mobile traders, or truck shops, it will take enforcement action if they do not improve their compliance.
Mobile truck shops are vehicles which drive around low-income areas selling bedding, furniture, clothing and other goods, typically at prices far higher than those found in shops.
A report released today said it identified 31 traders during the past year who did not comply with all of their legal obligations, under the Fair Trading Act and the Credit Contracts and Consumer Finance Act. after consumers complained about exorbitant prices.
These prices could be as much as $20 for a can of corned beef, or $35 for a packet of noodles.
They offer credit and do not require cash so have found success with desperate people who are short on ready money, not least as they do not do credit checks and go directly to people's homes.
They target those who may not have a working car or means of transport, making going to the shops difficult.
Once the goods have been bought, customers are expected to pay back the sum in weekly repayments of $20 or $30, which they are often unable to pay.
Commissioner Anna Rawlings said sales agreements, or credit contracts, were generally deficient.
Ms Rawlings said they often failed to outline basic details including what the goods are, the exact payment terms, the total amount owed and the end date.
Two traders are being investigated, while the remaining 29 have been told what changes they must make to comply with the law.