I understand that Japanese never planned but only discussed invading Australia. Navy thought 50k troops would do the job. Army escalated that figure to 250k & 500k of troops. I estimate a whole million based on islands coastline distribution of country towns & excellent infrastructure to allow owe resources to come back time after time.
History shows two large geographical features make an invasion difficult. 1) large archipelagos. Indonesia has 18k islands. Phillipnes has 14 k islands. Japanese navy would be stretched thin controlling those two geographical features. 2) Next large continental areas like Canada/USSR make invading armies stretch their resources very thin
3) Australia in its present context would be impossible for Japan, China or India to invade. That is why British were first here.
