Can't remember where I heard that explanation of "ocker" - I know I didn't make it up. I think it was more likely on the radio than anywhere, years and years ago.

The suggestion was that it started out as a way of putting down immigrants from England with a trades union background who were seen as stirring trouble - as I said, a bit bloody-minded. But then presumably it got taken up by Australians for some of their own, since being bloody-minded when need be is as Australian as it comes.

Of course it could be fake folk etymology - but it sounds quite plausible to me.

I found this definition on this site - Living in Australia: ocker - an uncultivated or boorish Australian; a knocker or derider.