rp trading at 1.60 per tonne., page-27

Yes, it is optional. This wording is from a Tax determination issued in 1994

"4. In the past, where unidentifiable «shares» have been disposed of, the Commissioner has accepted 'first-in first-out' as a reasonable basis of identification. For CGT purposes, the Commissioner will also accept the taxpayer's «selection» of the identity of «shares» disposed of."

However bear in mind that tax determinations, fact sheets etc do NOT have the force of law and the ATO has been known to challenge taxpayers who act in accordance with such guidance.