How do I find my adult son's father. Electoral searches don't help.?

I knew him as Errol Cook, but his birth name was Williams. He would probably be in his late 50s or early 60s now. Had a daughter born in 1977 in Wollongong and my son Steven was born Sydney 1979. If anyone thinks they might know him, Steven would so much love to meet him, especially as he is a dad himself to two boys. He would also dearly love to meet his half sister. Please help if you can. Errol did know about Steven, but has not seen him since he was a year old.