The Robodebt scheme was branded "cruel", "crude" and "neither fair nor legal" by a royal commission, but nobody involved in its implementation or operation will likely ever face criminal charges.

Yesterday, the National Anti-Corruption Commission released the findings of its investigation into six people referred to it - by the royal commission.

While four - including former prime minister Scott Morrison - were cleared by the NACC - two public servants were found to have engaged in "serious corrupt conduct".

But the investigation concluded there was "insufficient admissible evidence" to refer them for criminal charges.

• GUEST: Dr Christopher Rudge, socio legal scholar and lecturer at Sydney Law School