Hi,
Go to :-
www.dhhs.vic.gov.au
..and search for Restrictions for those in Melbourne/Mitchell Shire. You will eventually get to a page detailing the rules for golf, surfing, fishing and boating. No explicit mention of planes, of course. However, it says you can do those activities provided you keep socially-distanced (eg 2 people playing golf must remain 1.5m apart and others should be about 100m away) and they must be done within the lockdown area. It says not to travel a long distance if they can be done close to where you live, but if your boat is moored 50kms away at the coast then obviously you will need to travel there to sail it. By implication the same must apply to your airplane - so you can travel to where the plane is hangared provided it is within the lockdown zone. No landing outside the lockdown area, of course.
I think they realized from the last time that as these sorts of activities are outside, and easily done solo, or many metres apart from others, the risk is tiny, and the benefit is huge for mental health.
I have this page bookmarked on my phone, so if an over-zealous cop questions it I can show them.
HTH,
Neil