Apologies for the late response. As you pointed out, there are no surplus hippos in Japanese zoos. Most facilities do not have the infrastructure to house hippos in herds, and they are typically kept in pairs (1.1) or a single female (0.1). When breeding is planned, the zoo must secure a destination for the potential calf before pairing, ensuring that no surplus individuals will be produced. Therefore, it is not realistic for Australian zoos to import hippos from Japan.
On the other hand, the breeding of pygmy hippos is encouraged in Japan, and currently, three pairs are being paired for breeding. There are already three surplus female individuals, and in the future, Hiroshima Zoo plans to secure a 1.1 pair domestically. However, since there are currently no surplus male individuals in Japan, it is expected that males will be produced from these pairings. Unlike hippos, it seems that more pygmy hippos may become available for export overseas (though Japan has never exported any pygmy hippos), but since the surplus individuals are likely to be females, they may not be useful for Australian zoos. Therefore, importing males from Singapore, Europe, or the United States may be the best solution.
Personally, I believe that Japan's major public zoos, particularly those that house koalas, have an obligation to respond to Australia's requests, considering the history of receiving many animals from Australia. There have been very few instances of Japan sending animals to Australia, mainly due to the difficulty of exporting animals to Australia. However, I strongly believe that Japanese zoos should repay the favor for the koalas they have received by engaging in large-scale projects, such as sending hippos or pygmy hippos. That being said, it is easy for an outsider like me to suggest, and I understand that it is a very difficult and unrealistic task.