Huw,
Chapter 7 of "Queensland Postal History", H. M. Campbell contains this info:
Registration introduced when NSW administered Qld, Qld Admnistration continued the system.
to 30 June, 1873 the fee was 6d for internal mail (I don't know what 'internal' means, whether within Qld or within Australia).
from 1 July, 1873, the fee reduced to 4d
from 1 October, 1891 the fee reduced to 3d (UPU rules).
Rules for international mail were different.
The letter rate was 1d from 1.5.11 so your letters are paying 1d postage plus 3d registration.
John
The book is still available new from the publisher for $AUD35:
http://www.rpsv.org.au/publications/