The ATO, which has paid $11.5 million in taxi fares for its employees in the past three financial years, says it is exploring "alternative and emerging modes of transport" as it strives to save money.

Tax had a $4.3 million contract with CabCharge in 2014-2015, according to documents lodged on the federal government's AusTender website but a spokesman said that about $3.8 million had been spent during the year taxiing ATO public servants around.

In 2013-2014, $3.6 million was spent in fares and the metre ticked over at $3.9 million in 2012-2013.

Various methods are being used to reduce transport costs as the ATO, along with other Commonwealth departments, feels the squeeze of shrinking budgets.

Uber has gone from a standing start in Australia to having 15,000 drivers, 1 million users and has provided 5 million rides posing a serious threat to the established taxi industry in the cities where the new service is operating.

But Uber is also proving a nightmare for transport and taxation authorities around the world.

The service and the ATO are locked in a dispute, which is heading for the Federal Court, after the Commissioner of Taxation ruled Uber was a taxi service and was liable to pay GST on its sales.

But despite the differences between the two outfits, when asked if it would consider using Uber to bring down costs, the ATO did not rule out such a move.

"The ATO continues to explore the use of alternative and emerging modes of transport," the spokesman said.

He said that other money-saving transport measures were being encouraged among the Tax Office's 18,000 officials.

"The ATO continues to maintain a focus on reducing travel costs and encourages (the) use of teleconferencing, video-conferencing or Microsoft Office Communicator in lieu of air travel."

Public servants were also being encouraged to arrange multiple meetings on business trips, share taxis to and from airports, use trains and buses and think about fleet vehicles or "other providers", the spokesman said.