The $47 billion in deductions claimed against rental income every year will face tougher scrutiny from the Australian Taxation Office after more than 300 audits of investor returns revealed errors in nearly 90 per cent of cases.

More than 2.1 million Australian taxpayers declare rental income to the Tax Office each year, but a check of returns has unearthed widespread errors in the popular tax deduction.

"A lot of people are getting things a little bit wrong, which adds up to a lot," Tax Office commissioner Chris Jordan told The Tax Institute's national convention in Hobart on Thursday.

Australian Taxation Office commissioner Chris Jordan has warned property investors to be careful with their tax returns. Credit:Wayne Taylor

Mr Jordan warned that property investors were now his "next focus", following a successful crackdown on inappropriate work-related expenses, which has yielded $600 million in extra tax revenue. For the first time in nearly a quarter of a century, the average work-related expense claim fell by, on average, about $130 a year for the past two years.