A criminal ring has this month stolen 1600 records from an accountant in the Australian city of Melbourne, says Fairfax Media.
The attacks relate to a phishing campaign detailed by El Reg against users of the Xero accountancy software.
Sources revealed businesses in NSW and Victoria had lost thousands of dollars due to bank accounts being changed from fraud.
Australia's Federal Police is also investigating.
Fairfax's report suggests fraudulent tax returns were lodged using some of the names found in a cache of 1,600 stolen personal records that included the names, addresses, dates of birth, and tax and bank details of staff employed by clients of the unnamed accountancy.
Figures in that story alleging some 500 tax returns are being ripped off each day by a criminal group are said to be inaccurate, however.
Some 26,000 tax returns have been delayed in recent months while 1000 returns clocking A$9 million due to fraud, Fairfax reports.
The Register has inquired with the Australian Tax Office regarding whether victims of fraud will be able to re-lodge their returns after criminals have already done so. In 2011 this reporter found victims whose tax returns were lodged using stolen identity credentials had no way to legitimately claim for that year.
The Office had at the time regarded those lodgements as being made since criminals had satisfied its 100-point identity check.
Tax thieves were using stolen identity information and going through accountancies including H&R Block. This reporter was able to submit his tax return through one major Sydney accountancy without having to produce any identification.
The ATO says in a statement most tax fraud occurs using stolen identity information.
"The majority of refunds that are claimed fraudulently are claimed using illegally obtained personal information and we would like to remind people to be vigilant with their online security," it says.
"We have intelligent systems and sophisticated analytics that automatically detect tax returns that are suspected of being lodged fraudulently using another person's stolen identity." ®