The "big four" are the unseen hand guiding Australian life.

Not the banks, but the global consulting firms KPMG, Deloitte, PricewaterhouseCoopers (PwC) and Ernst & Young (EY).

Their revenue in Australia topped $7 billion last financial year, and the sector is growing at nearly 15 per cent annually — powered by rampant acquisitions of everything from advertising agencies and marketing companies to law firms.

"The firms are incredibly important in Australia — they're proportionally larger here than anywhere else," Stuart Kells, co-author of a new book The Big Four, said.

"And they've got fingers in all sorts of pies: corporate auditing, government advice, not-for-profits, real estate, marketing, you name it.

Those global consulting firms are no longer just checking the company books.

Although traditional audit and accounting roles remain, their spending sprees have extended their reach into all facets of commerce and government.

"The big four characterise themselves as multi-service firms, and they generally don't say no to anything," Mr Kells said.

"If you come to them with an opportunity in marketing, real estate — they'll be up for it."

Outperforming the economy

KPMG has "enjoyed almost double digit growth" for the last five years, according to its chairman Alison Kitchen.

"With the economy growing at about 3 per cent during that time, clearly that cannot continue indefinitely or else we'd end up taking over the world."

The firm's role in helping Australian governments provide services is controversial, topping $500 million in the last financial year.

"A much wider range of the society uses us," she said.

"So whilst in some countries we work with [only] business, here [in Australia] we work with business, the not-for-profit sector and the government sector, providing all of those businesses with services to help them innovate and grow."

That move, beyond what she calls "historic tax and audit", is part of what has super-charged the growth of the big four consulting firms in Australia.

Branching out far and wide

PwC was to blame when its accountants handed the wrong Best Picture envelope to Warren Beatty, the host of the 2017 Oscars. ( Reuters: Lucy Nicholson )

Deloitte has taken over more than 28 companies in the past four years — everything from an identity security company to a virtual reality illustration firm.

EY has snapped up a tax law firm, two data analytics units and a market research firm, among others.

PwC has purchased companies which drive infrastructure projects, plus a stake in an advertising agency.

As for KPMG, it made 16 acquisitions in the past four years — from engineering to market research.

"Whilst I don't think that we are 'too big to fail' … I'm sure that the regulators are not going to allow us to get ourselves into that position," Ms Kitchen said.

"I do understand that it's highly unlikely, for example, that further consolidation in the sector can occur."

The firms operate in plain sight — until things go wrong.

For example, when PwC tallied the votes for the 2017 Oscars and gave the wrong envelope to the hosts, who then mistakenly announced La La Land (instead of Moonlight) was the Best Picture winner.

That is the strength and weakness of the firms, according to The Big Four's co-author Ian D Gow.

"Nobody knows that PwC is tallying the votes until they get it wrong," he said.

"It's symptomatic of a lot of their business — if they're doing audits right, no one pays attention.

"But if things go wrong, that when's they attract attention."

Disrupting the future

The firms are not listed on the stock exchange and do not have to release financial reports.

There are almost 2,500 partners in Australia, with more than 10 times that number of employees, and their impact is huge.

But "disruption" is the force dominating our time, and the firms are not immune.

Their increasing breadth comes as automation threatens to wilt profits from their most lucrative fields.

"We see fundamental pressures in all the service lines," Mr Kells said.

"Tax, advisory, audit, or some of the newer areas like law — all are subject to their own pressures. The rate of growth may well … stop".

Ms Kitchen said she believed KPMG was not ready to "take over the world" per se.

"We've done a great job of working towards disrupting ourselves, and that makes us well-placed to continue to help other firms disrupt," she said.

But Mr Kells has a slightly different view about the future.

"Think of them like a hothouse experiment in how far you can move away from those professional roots — high integrity, human scale.

"How far can you go to this arc of ultra-commercial, like investment banks where you chase down whatever [opportunities] you can?"

Given the breadth of industries in which the big four operate, if it is just an experiment for them, then we are certainly the subjects.