Australia
Paquebot at Auckland, New Zealand

This Australian aerogramme (Stein no. A-6) was posted in international waters on board R.M.S. Oronsay on its way from Sydney to Vancouver and fed into the postal network at Auckland, New Zealand (cds AUCKLAND PACKET BOAT, 4 JA54).

"1954   January 4   The Orient Line's new 28,000-ton luxury liner Oronsay berthed at Auckland at 7am today on her inaugural voyage reopening the Pacific service from Sydney to Vancouver and San Francisco.  The service was stopped when the 30 year old Aorangi went to the breakers' yard last year. " [THE NEW ZEALAND MARITIME RECORD]

According to the 1952 Universal Postal Convention, "In the absence of other arrangements between the Administrations concerned, the postage on correspondence posted on board ship on the high seas may be prepaid by means of the postage stamps and according to the tariff of the Country to which the ship belongs or by which it is maintained".


R.M.S. Oronsay was owned by the British Orient Line (Orient Steam Navigation Company), so British stamps and/or aerogramme forms should have been used when posting at the high seas. 

Although the sender was the renowned philatelist G.J. Hutson (author of "The Stamps of New South Wales"), the aerogramme with respect to its contents, a private message, is not ostensibly philatelic.


References
More about THE NEW ZEALAND MARITIME RECORD
Stein refers to Stein,The Aerogrammes of Australia and its Dependencies 1944-1980