Aas long as you leave a country with the same passport that you entered in on, then you're ok.
- Option 1: Entering Aus on Aus passport, leaving on Greek = bad
- Option 2: Entering on Greek, leaving on Aus = bad
- Option 3: Entering and leaving on Greek passport = good
- Option 4: Entering and leaving on Aus passport = good
The reason being for counts and tracking - if a Greek tourist has entered Australia, but has not been seen leaving, panic ensues (well probably not, but that's the theory anyway). That passport has 'overstayed'. And you NEVER want to get pinged for overstaying.
Once you've left a country, meh, they no longer care about you. So when you come back to the UK, you can use whatever passport you want, so long as that passport has the legal entitlement to work there (if you're working there). Greek passport is obviously easy for this, but the Aussie one you'll require a visa of some sort, if you present that at the border, as you're now being treated as a non-EU citizen (as that's the passport you chose to present).
Hope that makes sense.