Bill Shorten is set to reinstall South Australian right-wing powerbroker Don Farrell to the Labor frontbench to fill the positions vacated by Mr Shorten's former Victorian factional ally Stephen Conroy.

Three Labor sources confirmed to Fairfax Media the changes would be announced on Tuesday, the same day the caucus is due to formally announce it will reject the government's bills to hold a plebiscite on same-sex marriage.

Known in political circles as "The Godfather," Senator Farrell was promoted to the ministry in the last Labor government by former prime minister Julia Gillard, but his career hit a roadblock when he was forced to give up his number one spot on the ticket for left-wing rival Penny Wong ahead of the 2013 election. He subsequently lost his seat.

After a two-year break running his own winery in the Clare Valley, Senator Farrell was reinstated on Labor's Senate ticket and re-elected in the 2016 poll.