With baggage to re-check or without?

I expect the BA flight uses a gate in the satellite terminal meaning you'd need to take the train over to main terminal area (where the OZ flight will depart) and if needed go through immigration to get bags and get landside for check-in.

If no bags I assume you can try to use the transfer line in the main terminal, but I think you need to show a boarding pass to go this way (transfer desks seem to be after security screening which has always puzzled me).

If you get strong head winds during the BA flight you could find yourself arriving 30 mins behind schedule which leaves you with little to play with.
Not sure I'd do it on separate tickets, but obviously different people have different attitudes to risks.