The former Celtic hitman has yet to score a single goal for Australia in 25 appearances.
Williams said: "He's a brilliant player. I think once he gets that ***** off his back, I think he'll start putting them in the net all the time – he's that type of player.
"I think he's a lot on confidence and that first goal, when it comes, will be massive for him and also the country.
"I just hope that he can get that goal, I know he's capable of it and he's such a good player that I'm surprised it hasn't come a lot sooner."