I am not sure if this could fit in with what you have in mind.
Palimsest letters were the result of the high cost of postage and form an interesting part of any Postal history collection (this is to the Cape of Good Hope). The writer wrote first horizontally, then turned the page through 90 degrees and continued vertically. This way a single page instead of two could be sent, with considerable savings in postage, as the single rate postal rate applied in this case.
The Ship Letter Act of 1815 (55 Geo. III C, 153), passed on 15 July 1815, was termed by some 'an act of preventing the correspondence of parents and their children in the Cape and the East Indies'. The rate of postage for a single packet letter was tripled to 3s. 6d. and a double letter was charged at 7s. This was an enormous amount of money in those days.
I have 3-4 I can part with one at the right offer. If you interested please send me an email and we can discuss.
They were all send to the wife of Maclear. In 1833, when the post became vacant, he was named as Her Majesty's Astronomer at the Cape of Good Hope, and arrived there aboard the Tam O'Shanter with his wife and 5 daughters, to take up his new duties in 1834. He worked with John Herschel until 1838, performing a survey of the southern sky, and continued to perform important astronomical observations over several more decades. The Maclears and Herschels formed a close friendship, the wives drawn together by the unusual occupations of their husbands and the raising of their large families. Mary Maclear, like Margaret Herschel, was a noted beauty and intelligent, though suffering from extreme deafness.
I am sure I saw similar letters from Australia.