The envelope, or so-called balloon post cover, was flown out of Paris in 1870 by hot air balloon when Paris was under siege from the Prussian army in the Franco-Prussian war.
It has been bought for by a French buyer for a record price of 190,800 dollars.
NSW manager Danny Jurd of Mowbray Collectables in Sydney said that sale had broken a record for any philatelic item sold in Australia.
He said the envelope is the only one of its kind addressed to Australia, making it the earliest known letter sent here by some form of airmail.
"The balloon post covers themselves are very collectable," News.com.au quoted Jurd as saying.
"To have one addressed to Australia was the unique part and the rarity of the item," he added.