Most useful languages other than English in Asia and Oceania [closed]

In all of my travels in Asia, the only places where I found speaking English wasn't as convenient as speaking the local language, or almost so, were Japan and China. Everywhere else, comprehensible English speakers were thick on the ground.

If you go to some country so small the welcome-to signs are printed on both sides -- say, Laos or Bhutan -- everyone knows you aren't going to speak Lao or Bhutanese, so they learn English. In China, someone who only speaks the local lingo has a billion other people he can talk to instead of you, so they spend that time, I dunno, uploading road-rage videos to YouTube or something.

So I would say: learn Mandarin. If you speak speak English and Mandarin, everywhere on the mainland of Asia and in the Southeast Asian archipelago, you'll be perfectly fine.

If by Oceania you mean Pacific Islands outside Meganesia, add French to your list. All of French Polynesia, plus New Caledonia, speak French. Vanuatu has many English speakers, but most I met also spoke French or told me their French was better than their English. Also the native languages through Melanesia and Polynesia are very similar. Learn some Maori in New Zealand and you'll be understood from New Caledonia to the Cook islands and on to Tahiti and the Marquesas.

However your question seems to focus on Asia, which is a different story. Your criteria of "close to Australia" applies to New Caledonia, Vanuatu, and Fiji for sure. East of Fiji, New Zealand accents outweighed Australian ones, but there were still Australians all the way to the Cooks.

I learned Mandarin, I have used it in many places I didnt expect to, because there are Chinese people everywhere.

I have used it in Australia (lots of Chinese students), Vanuatu (with shop keepers and restaurants), Hong Kong (reading only, good for menus and signs), Svalbard (lot of chinese tourists).
But.. using it in China it really pays off, it is a hugely diverse country and for a traveler who can be bothered to make the effort to learn very rewarding.

Hindi along with English is mostly spoken in the South Asian countries mainly in most parts of India, Bangladesh, Bhutan, Maldives, Nepal and Pakistan and especially in all tourism areas in these countries.

While English is understood by most of the locals in these countries speaking in Hindi can be helpful to you as you would sound like a native even though you may look like a foreigner. Speaking in Hindi can be helpful in bargaining while shopping and can prevent you from getting ripped off by the local businesses especially in tourist areas.

Chinese. Chinese is the most widely spoken language in the world. And yes, probably Mandarin, since Mao decreed all Chinese would standardize on it (which was only mildly successful, but whatever.)