Money paid to the big four consulting firms has doubled since the Coalition came to office, at a time when public servant numbers have tumbled.

Tabled to a Senate committee investigating the quality of auditing in Australia, the figures tell a tale of just how much work is being outsourced to the private sector under the Coalition.

KPMG has become a big winner under the government, raking in $262 million last financial year, and more than $800 million over the life of the Coalition government.

In the same period, the company's political donations to all parties have grown from $105,000 in 2012-13 to $190,540 last year.

PriceWaterhouseCoopers came in second as far as government work, making $191 million last year. The company donated significantly more than other consulting firms, at $386,635 for the year.

Deloitte took home $179 million last year while Ernst and Young made $87 million from the government.

According to documents filed with the Australian Electoral Commission, slightly more funds were donated to the Coalition than to Labor, but it is also shown that three of the big four - PriceWaterhouseCoopers, Ernst and Young and KPMG donated more to Labor in the lead up to last year's election, when a change of government was widely tipped.

These figures only tell part of the story of government outsourcing, which also includes labour hire and contractors. Despite repeated efforts to capture how many people are employed as contractors or consultants to the government, and how much money is spent, this information isn't tracked centrally.

Finance Minister Mathias Cormann is a strong proponent of the government's use of consultants, telling a Senate estimates hearing last year that there hadn't been an explosion in spending on contractors because the overall cost of government administration had remained steady.

Greens Senator Peter Whish-Wilson, who requested the big consulting firms reveal just how much they had made, said it showed just how cosy the relationship was between donations and winning taxpayer-funded contracts.

"In a closed loop, we're seeing huge donations to the major parties by the Big Four and lucrative public contracts in return," he said.

"It has to be said that it's a pretty good return on investment for them."

The hearings are part of a larger look at the quality of auditing including of big companies.

"The hearings raise fundamental questions about not just their independence, but the quality of the Big Four's auditing work, their corporate structure and their role as political players," Senator Whish-Wilson said.

"Do we have a case here of yes, minister, in which these private providers can be relied upon to tell Cabinet what they want to hear?"

The number of public servants reached a 13 year low last year, dropping to 147,237 people.