From the Ukrainian frontlines to the mines of the Central African Republic, this week's Four Corners lifts the veil on Putin's private army – The Wagner Group.

In this independent French television investigation, former senior Wagner insiders speak out about what they saw in the line of duty.

The film traces the group, from its origins in Ukraine, to its clandestine operations in Syria, and its reign of terror across Africa where victims tell of rape, torture and massacres.

It also follows the rise and fall of the group's leader Yevgeny Prigozhin who recently dared to challenge Putin's power by marching on Moscow with thousands of his troops.

This is a story that Angus Grigg has wanted to tell for five years.

It's got all the elements – money, insiders and power – but until recently, the rise of the consulting giants and how they've infiltrated government was confined to the business pages.

Then the PwC tax scandal happened.

This week on Four Corners, we investigate PwC's equally large and consequential rivals, and their huge influence in Canberra.

Top of our list is KPMG, which has billed the Department of Defence a staggering $1.3 billion over the past five years.

Whistleblowers tell us how it repeatedly over-charged the department – and you the taxpayer – for work never completed, and even invoiced for a consultant who wasn't on a project.

And we don't stop there.