CANBERRA, April 2 (Xinhua) -- The value of Australian tax credits held by global mining giants grew to 324 billion Australian dollars (230.2 billion U.S. dollars) in financial year 2017-18.

The Australian Taxation Office (ATO) on Tuesday revealed that the value of tax credits, which companies can use to lower their tax liabilities in future years, rose from 282 billion AUD (200.4 billion USD) in 2016-17.

According to ATO data obtained by the Australian Broadcasting Corporation (ABC), of 138 tax returns lodged by oil and gas mining companies - only six "profitable" projects paid the petroleum resource rent tax (PRRT) - a tax on the sale of natural gas and oil - because of tax concessions.

The scheme, which was introduced in 1987, was updated in 2012 to apply to all oil and gas production, including shale oil and coal seam gas.

Under the concessions included in the PRRT, companies whose spending exceeds their assessable tax receipts receive a credit which can be carried over year-on-year.

Peter Whish-Wilson, the Treasury spokesperson for the Greens Party, said that the 324 billion AUD equates to 70 percent of the government's total annual revenue.

"The PRRT is the most egregious rort in the Australian tax code," he told the ABC.

"While the world is in the middle of an LNG boom, we're practically giving the stuff away.

"On current trends, it is an open question whether PRRT tax offsets will ever be expired.

"Some companies might ride a multi-decade long boom and end up tax positive."

Whish-Wilson's view was disputed by Stuart Robert, the assistant treasurer, who said that the tax credits are "a function of the money actually spent by companies" in Australia.

"Tax losses, or credits as they are sometimes called, represent money companies have actually spent on developing projects, plus uplift, and are mostly not transferable between projects," he said.

"As a result, the aggregated carry forward PRRT tax losses published by the ATO are not directly relevant to whether individual projects will pay PRRT."