We just returned from Tasmania and found many activities that were wheelchair/mobility scooter accessible.
We took our van over which meant I could use my own scooter.
There were a number of accessible walks:
Fernglade Reserve near Burnie – where you can go platypus spotting
Gordon River cruise (from Strahan) – accessible boat and accessible walks through the rainforest and onto Sarah Island.
Franklin River – accessible walk en route to Lake St Clair
MONA – accessible boat to an accessible landing and easy to get around MONA
Cape Tourville – Freycinet National park has an accessible walk up and around the lighthouse.
Cataract Gorge, Launceston- accessible but be careful on some of the slopes.