In my Post Charges easy guide within Australia  As at 8 April 2013 it says:

Compensation up to $50 may be paid at Australia Post’s discretion upon production of a receipt or other proof of lodgement for loss or damage.

This is in the Letter Post section (Large letters by ordinary post)

Whether that is enough to help you if something untoward happens, is anyone's guess

Like you I haven't had to put in a claim for a number of years

Maybe find out what the current Registered Post Prepaid Envelopes say on them about contents. The B4 one might be overkill size wise but allows a thickness of 20mm, a charge of 'only' $5.10 and compensation up to $100 (my old style Registered Post Prepaid Envelope does NOT say contents are limited to papers/printed matter)