Hmm…
https://www.ebay.co.uk/itm/135560427820The vendor understandably misread the postcard caption as Kaapai- the 'K' is a capital H.
Ha'apai is a group of islands; there doesn't appear to be a particular island of that name. Lifuka is the largest island, and has the administrative capital, Pangai.
So that's my best guess
There appears to have been a small number of Japanese living in Tonga in the 1930s, including married couples and men married to Tongans. On the outbreak of hostilities the Japanese were removed to New Zealand with the intention of sending them to Australia prior to a planned exchange of civilians with Japan. This didn't take place- antipathy towards the Japanese led to this group's postwar return to Japan.