Blackhand is correct, you'll need a maintenance authority under Reg 33B(1)(a) to fill the bottles fitted to the aircraft.
If you carry an external bottle, this is different, it is then considered baggage/freight, and due to the contents being over the 30 odd psi magic number it is a DG, but talk to the CASA DG guru, I believe there is either an exemption, instrument or ruling to carry these without consignment.