The club have been linked with the 27 year old Australian, who travels on a Maltese passport, exempting him from the overseas quota, for several weeks.
Branighan, a former Australian Schoolboys starlet whose early years with St George and Cronulla were ruined by a succession of knee injuries, partnered Fax’s livewire number seven Ben Black in a succession of New South Wales junior sides.
Fax coach Matt Calland said: “I’ve been very impressed by Luke this year.
“He’s got an excellent kicking game, which is just what we need in terms of building pressure on teams.
“He’s scored 20-odd tries and he’s very smart with the ball in hand, he spots gaps very well and puts people into them.
“He’ll work really well with Ben: one’s a matchwinner and the other is a genuine organiser who can move a team round the park.
“I just think it’s a very exciting signing for this club.”