The revelation Bruce Billson received undisclosed payments from a business lobby group while still a serving MP has raised eyebrows among his party colleagues, but the former cabinet minister insists he acted with complete "transparency and integrity".

Mr Billson, who retired from Parliament at the 2016 election after being dumped from the ministry when Prime Minister Malcolm Turnbull took over as leader, has said he committed a "discourtesy" by failing to declare an income from the Franchise Council of Australia in his parliamentary register.

Retiring MP Bruce Billson (left) during the 2016 election campaign, with Liberal Chris Crewther, his successor in the seat of Dunkley. Credit:Andrew Meares

The former small business minister announced he was taking up the new role in March 2016, ahead of the July 2016 election, but then began receiving an income that was not disclosed on the parliamentary register, the ABC's 7.30 program revealed on Tuesday night. He is now facing demands to donate the amount to charity.

A Liberal MP, who described Mr Billson as a friend, said: "You can line a job up while you're still in the parliament but it's another issue altogether to start pocketing the coin."