He was last seen by his sister at 8pm on Saturday August 18, and was reported missing after he did not return from the toilet.
The 44-year-old suffers from an intellectual disability which requires medication.
Mr Williams’ family told police he could be in the Liverpool are or in the Villawood area where his mother lives.
They said he is also known to wander around parks and railway stations.
Mr Williams is described to be of Aboriginal/Torres Strait Islander appearance, with a large build, weighing about 120kg, and having short black hair.
At the time he went missing, he was wearing a black, zip-up jumper over an orange t-shirt, blue jeans, grey shoes and a blue baseball cap with a superman 'S' on the front.
Police said anyone who sees Mr Williams is advised not to approach him, but instead call Triple Zero.