Thanks for the responses.
So it turns out it is on my record. Unfortunately, I have been in and out of Australia twice since then which means I have answered "No" incorrectly on my Incoming Passenger Card (IPC). This was a genuine mistake as I didn't realise i had a criminal conviction.
I was planning on applying for PR as i am now eligible. I have been reading a little around the implications of a criminal offence such as this and the chances are it wouldn't prevent me from getting PR. However, the fact that I have not declared on numerous occasions flying into Aus it may turn it into a much bigger issue as it looks as though I have lied, and therefore raises questions about my character.
What I am thinking is going to them now and explaining the situation and that I made a mistake.
Any thoughts?