Australia's Commissioner of Taxation has flagged a crackdown on multinationals, saying the ATO has run out of patience with six large corporations which have been avoiding tax.

Key points: Taxation chief says big corporations gaming the system

Taxation chief says big corporations gaming the system Six un-named corporations now on high-risk list

Six un-named corporations now on high-risk list Tax bills on the way as soon as talks finish

In his opening statement to a Senate estimates hearing, Chris Jordan rounded on companies he said had been deliberately "gaming the system" to doge their tax obligations.

He told the committee the ATO had moved six large corporations into its "high-risk" list, and written to another 60 companies demanding they get their houses in order.

Mr Jordan said he would bring protracted tax negotiations with companies to a swift conclusion and issue them tax bills as soon as possible.

"These companies have pushed the envelope of reasonableness — they play games, they string us along, they believe we can be stooged. However enough is enough — no more of this," he said.

"How is it possible that companies known for their new-age technology and innovative products and services, fail to be able to furnish us with basic reports showing their business structures, their profits, how much tax they've paid and where?

"We are ruling the line under these protracted negotiations and proceeding immediately to raise assessments and create liabilities on these cases — potentially taking them all the way to court if necessary."

Mr Jordan did not say which companies had been moved into the highest-risk category.

But, he cited recent legal victories against Chevron and Orica as evidence "the worm is turning" in favour of the ATO.

"Cases featuring the same types of rolled-up loans and intra-company financing arrangements will be aggressively pursued," he said.

"I expect significant assessments as a result."

Mr Jordan also said a former Federal Court judge had been appointed to help design and test the legal settlement process.

"The ATO will not settle a dispute at any price," he said.

"This independent examination of our settlement ensures that the best outcome is likely to be achieved, and that we are settling the right cases in the right way.

Mr Jordan said the ATO was currently conducting 70 audits of large businesses, and now had more than 150 people working in the group focused on tax paid by international companies."