The Labour Party says the taxpayer has paid up to $500 million too much because the Government rejected bids for failed company South Canterbury Finance.
Finance Minister Bill English has refused to disclose details of offers for the business, which has gone into receivership.
The company's 35,000 investors will get their money back under the Government's Retail Deposit Guarantee Scheme.
Labour finance spokesperson David Cunliffe says he has seen documents that appeared to show the bids were genuine and credible.
Mr Cunliffe says one bid was for $1.3 billion and the Government must explain the loss.
He believes the offers may have been rejected out of ideology.