Prime Minister John Key says he's not sure why more mobile truck shop operators aren't being prosecuted.
A year-long Commerce Commission investigation into 32 traders found 31 of them were in breach of the Fair Trading Act, Consumer Finance Act and the Credit Contracts Act, but only a couple are being investigated further.
"Why they're not prosecuting all of them, I don't know," Mr Key said on the Paul Henry programme this morning.
"But ultimately… people need to be held to account."
Labour has called for truck shop operators to be licensed, so their right to do business can be simply revoked.
"One of the easiest things to do is get in a truck and go around and sell goods – you don't have to be licensed, there's no checks on you, you're preying on the vulnerable," he said last week.
Mr Key says the Government will look into why the Commerce Commission isn't taking a harder line.
"The law was changed very recently, in the middle of this year, to allow them to have the tools to [prosecute]. We'll have to go away and look at why they're not taking cases."
Mobile trucks shops often charge incredibly high interest rates, meaning while low-income families can acquire goods they can't afford to buy outright, the amounts owing quickly snowball into debts they can't repay.
"Nappies that they might be able to buy down the road they'll buy off these guys for two or three times the price or whatever it might be, then with all of these interest rates, it balloons out to a ridiculous price," says Mr Key.
But he doesn't think capping interest rates, as is done in other countries, will work.
"If interest rates are capped at a certain percent, then that will become the target; and because the lending risks are greater, in theory, you wouldn't make it 10 percent like the bank or 8 percent or something. You'd have to put it at some reasonable margin for them, because they're small amounts and there's risk and all the rest of the stuff.
"So where would you set it? If you set it at 24 percent, then you end up in a position where these trucks go around and say, 'Oh, the law says the interest rate's 24 percent.' Now that wouldn't be right, but people might be duped into that."
Labour's Kris Faafoi has drafted a Private Member's Bill that would cap interest rates.