UK postage used in Western Australia

Why might UK postage be used in Western Australia in April 1940? I have attached a copy of the front of the cover.
Here is a description of the cover
Paquebot cover (in German language) posted from Fremantle, Western Australia on 2 April 1940, using Great Britain stamps destined for Deurloostraat 120, Holland. The cover was censored in Perth, Western Australia. both label and handstamp applied. Once the cover reached Europe, it was at some point taken off the train, and held until the end of the war (see “Background” information with previous cover). In 1948, the cover was reposted to Holland, arrived in Amsterdam April 4, 1948. When forwarded to the addressee after eight years, the a handstamp indicating “does not live at that address…” (handstamp applied on back), along with “unknown” marking.
Here is a description of the cover
Paquebot cover (in German language) posted from Fremantle, Western Australia on 2 April 1940, using Great Britain stamps destined for Deurloostraat 120, Holland. The cover was censored in Perth, Western Australia. both label and handstamp applied. Once the cover reached Europe, it was at some point taken off the train, and held until the end of the war (see “Background” information with previous cover). In 1948, the cover was reposted to Holland, arrived in Amsterdam April 4, 1948. When forwarded to the addressee after eight years, the a handstamp indicating “does not live at that address…” (handstamp applied on back), along with “unknown” marking.