In the eyes of Aus Post I thought parcels and letter were treated differently. I am fully away you can simply purchase tracking and signature for parcels, but this option is not currently available for items under the large letter category, "from what i have been told by our Local Aus Post Manager"

I've been sending large letters Registered for ages and sent one as late as last week. Not sure if a signature will be able to be purchased for a large letter after April 8th but we'll just have to wait and see. Everyone will be in the same boat as far as that goes, so it should even out amongst sellers anyway.