The Commonwealth child support system, which is supposed to support more than 1.2 million Australian children, has fallen victim to the latest federal government tech wreck.

A $104 million project to replace the ageing technology which supports payments of more than $3.2 billion a year is late and in disarray, with about 3000 public servants at the Child Support Agency still trying to work with technology that was declared obsolete four years ago.

A Senate inquiry will probe the government's troubled multibillion dollar spend on IT projects. Credit:Phil Carrick

The giant Department of Human Services, which runs the agency, pledged to have a new system up and running by mid-2016, but despite being given $104 million to spend on the replacement, cannot say when the job will be done or how much of the money has been spent.

Human Services, which has been plagued by a string of failed IT projects, is the department responsible for the troubled myGov web portal and has been entrusted with $1 billion to replace the main welfare payment system in a giant undertaking called WPIT.