The BMW nav OS does not support encryted TMC AFAIK.

On the BMW system, the radio module receives the TMC data and sends it to the nav computer, in its raw received format

The nav computer takes the TMC data, decodes it, and then reads the TMC location database on the map disc to find out the lat/long of the location IDs

So you see a few things have to be in place for TMC to work:
1. The radio module must have the TMC receiver built in (not all radio modules do)
2. The nav must support TMC (all nav OS versions do, but some markets have the TMC menu option removed to stop users complaining that it doesn't work)
3. The location database for the country must be present on the map disc

If the TMC data is broadcast in an encypted format, then decryption must take place either inside the radio module or inside the nav.
To add decryption, the OS of the nav needs to be changed, or the radio OS has to be changed.
Neither are trivial or easy.

If the Australian system uses a conditional access system where the CA codes can be stored on the map disc, then maybe it would work.
You'd have to ask the Australian TMC provider as to what CA system they use.

But no matter what - without a TMC database on the map disc, TMC simply cannot work.

Also consider asking VDO Australia, because the nav system in the BMW is a VDO system. If VDO are considering adding TMC support, ask them how it is done, and if it can be made to work on the older VDO nav units (like MS3000, MS3100, which are very similar to the BMW nav units)