This example of a mailed leaf comes from the Puke Ariki Museum in New Zealand. I went hunting down an example after my pen pal Julia mentioned that a pen pal of hers sent her one of these leaves, albeit enclosed in an envelope. She told me that they were accepted as mailable postcards on Ulva Island until the 1950s. That's probably apocryphal as the post office on Ulva closed in 1923, but mail was still sent from the larger Stewart Island nearby. Since the leaves of the Muttonbird Scrub bush were pretty much the perfect postcard size, they became a favorite quirky souvenir to send from the islands.
Notably, the post office did put up regulations against it; a pamphlet of 1 March 1906 stated: 'The transmission of tree-leaves posted loose and bearing written communications to the United Kingdom or to countries in transit through the United Kingdom is forbidden'. In September 1912 it was again mentioned that 'Loose-tree-leaves are not to be accepted for transmission by post to any address'. Despite these warnings postcard senders were not discouraged and, in February 1915, another memo stated: 'Loose tree-leaves are prohibited, and if posted are to be sent to the Dead Letter Office for disposal'. However, most historians seem assured that mailed leaves got through the post long after that decree.