Red Jet,
73.2.11 Flights which cannot use TTF will plan the flight on the current TAF until such time as the destination ETA falls within the validity period of a TTF.
This means that you are required to update your preflight TAF with a TTF when you are airborne.
6.1.2 The issue of a new forecast by a meteorological office, such as a routine aerodrome forecast, shall be understood to cancel automatically any forecast of the same type previously issued for the same place and for the same period of validity or part thereof.
Self explanatory. You can't just use the first TTF you get 3 hours out from your destination, as the TTF issued 2.5 hours out cancels the first one. You essentially have to get the last TTF issued before your PNR.
I can further tell you that Australia is the only of those countries where I can even file an IFR flight plan without a nominated alternate, REGARDLESS OF FORECAST WEATHER CONDITIONS, so it is pretty clear to me that Australia has a variation to the recommendations stipulated in the relevant ICAO Annex.
No you can't. You can plan to a PNR based on a suitable airport, as you can to destinations in any other country. This is explained in this document
http://www.casa.gov.au/download/caaps/ops/234_1.pdf