The Newcastle forward has not started a game for new boss Kevin Keegan since he took charge seven weeks ago. And, Viduka's absence from the first team has fuelled speculation that he might leave the Premiership to wind down his career in Australia.
"I don't know of any approaches," a Sydney spokesman said on Saturday.
"As much as we would love a player of his quality and standing at the club it would be impossible to fit him in under the salary-cap restrictions."
Meanwhile, Melbourne Victory chief executive Geoff Miles insists the club is unlikely to secure Viduka's signature because it is likely he'll want to continue plying his trade in Europe.
He said: "I would think Mark [Viduka] is still several years away from playing in Australia again."
A spokesman from Viduka's management company insisted their client's future lies in Europe and not making a swift return to Australia.
"I'll be a monkey's uncle if Mark comes back to play in Australia in the very near future," he said.
"I think there is no doubt that Mark still sees his immediate future over there and is not yet ready to leave Europe. He still has a couple of good years left in him.
"If anything was to happen at Newcastle I'd imagine there would be clubs in England, Europe and Scotland which would be keen to sign him.
"But that's not to say he won't come back eventually and finish his career in Australia. I think that is very much in his plans."