The Australian Taxation Office has only one company in its highest risk category for tax avoidance and it's not a high profile technology business.

Four years ago, 13 of the country's biggest companies were in Q1, the ATO's highest-risk category for tax avoidance, but now only News Corp Australia remains, reflecting what is described as a secretive, aggressive approach to the ATO.

As the only company with a high-risk rating, News Corp faces continual audits and reviews.

News Corp declined to respond to questions by The Australian Financial Review about its tax-risk status.

On April 22, ATO officials at the Senate committee inquiry declined to answer questions by former Greens leader Christine Milne about the tax status of News Australia Holdings which holds most of News Corp's Australian subsidiaries. However, the Financial Review has confirmed with several sources that News Corp is the mystery company still in Q1.