We don't have design data for TTE hybrid products but it's possible Greg Nolan at the University of Tasmania might be able to help. Greg is a professor at the UTAS School of Architecture & Design and you will find more about his background via this link https://discover.utas.edu.au/gregory.nolan. 

Red ironbark is one of the seven timbers that have passed the test in AS/NZS 3837 to be classed as bushfire-resisting and therefore is suitable for external exposed elements up to and including BAL-29 (eg. wall cladding, decking, etc).

The probable service life of F14 unseasoned hardwood will depend on the species of timber, so it's hard to draw a comparison with H3 treated pine without knowing the hardwood's species and Durability Class. Using seasoned hardwood instead of unseasoned hardwood would eliminate, or at least greatly reduce, problems of twisting but at greater cost and reduced workability. H3 pine is typically treated by a LOSP process and the service life predicted in our Technical Design Guide assumes regular maintenance, avoidance of water traps and protection of end-grain.

An economical choice, and a timber that's easy to work with, is preservative treated pine. The two main treatments are CCA and ACQ. There has been some concern about using CCA-treated pine for vegetable gardens on account of its arsenic content, and presumably that would also apply to constructing compost bays. However CSIRO considers it is safe with some minor precautions - refer their data sheet titled The facts about CCA-treated timber, available on the net. If preferred, pine treated with arsenic-free ACQ could be used.