I would do 25% water changes every 2wks and do spot cleans of turds/leftovers regularly. Good idea to also get ammonia and nitrate tester kits cos these can build up quickly and can be toxic. A coloured basking lamp (in addition to UV light with no glass in front of it) aswell as a heater for the water will allow the turtle to dry its' shell and regulate it's body temp better reducing the risk of disease.
'turtle food' in australia is unregulated in terms of ingredients and nutrients. It is therefore better to feed a 'natural diet': Australian tortoises only feed in water and should be fed whole fish(not frozen), adult mice, yabbies, shrimps, worms, insects (inject or feed on calcium diet) and occasional dog food. Tortoises should be fed daily for jouveniles to three times weekly
for adults and less in the colder months.