He played at Adelaide from 1997-1998, Canberra from 1999-2001, Northern Eagles in 2002, Manly 2003-2008 and Harlequins from 2009-2010.
At Adelaide he played mostly centre, but he also played as a 5/8th, lock and winger.
At Canberra he played mostly centre, but also winger, hooker and lock.
At Northern Eagles he played 5/8th and at Manly he played second-rower, hooker, 5/8th, centre, lock.
You would assume he was signed by the Northern Eagles to play 5/8th - and someone was needed in the halves after Jason Ferris got injured early that season - but he certainly played most of his matches at Canberra in the centres.
I got the position info from: http://stats.rleague.com/rl/scorers/players/L/Luke_Williamson.html
He certainly was a true utility.