According to H&R Block research, 39 per cent of cases had missed out on refunds averaging $237.44 each.
In a further 20 per cent of cases, self-lodging taxpayers who either over-claimed refunds or under-declared income underpaid an average of $828.35 in tax, meaning the ATO has potentially missed out on $565 million in tax revenue.
H&R Block said this means that the taxpayers concerned could be looking at severe consequences, including an audit, repayment of the tax shortfall, or penalties of up to 95 per cent of the tax underpaid and interest at over 9 per cent per annum.
The research also revealed that 97 per cent of taxpayers who did ‘do-it-yourself’ returns were confident of their accuracy, but after an examination of a sample of self-lodged tax returns from last year, 59 per cent of them contained errors.
H&R Block director of tax communications Mark Chapman said that there is a real lack of knowledge around tax.
“Many people lodge tax returns themselves to save the accountant’s fee, or they are confident in their own ability and knowledge,” he said.
“But as our review shows, by not seeking professional help, people are potentially missing out on valuable deductions and putting themselves at risk of a costly ATO audit.”