Bill Shorten has sparked a new Labor factional brawl in his home state of Victoria after backing controversial lawyer and close friend Kimberley Kitching to replace Stephen Conroy in his party's plum top Senate spot.

Ms Kitching secured the party's backing for the seat on Thursday night with the active support of Shorten's union base, and others, after a frenzy of factional bargaining and lobbying through the day.

One major union, the National Union of Workers, boycotted the selection committee vote in protest at the lack of democracy in the preselection of Labor senators.

Ms Kitching, from the Victorian Right faction and the wife of former political blogger Andrew Landeryou, is a contentious choice, with senior party figures warning of the dangers of her preselection to the party and to Shorten.