Keep in mind Indonesia has a population of approx. 260+ million and Australia has 29 million. Even if over half of the Indonesians have phones their network coverage is extensive. They also live via their social media. So will you have coverage, yes of course you will but will the data be as we know it - maybe not. Their lines are very full of users, so data maybe slower than what you are use to, but you will still have it. Evenings are definitely slower than during the day on the network.
You must ensure your phone is unlocked prior to leaving Australia.
Interesting statistical data such as: Indonesia has nearly as many mobile phones as it does people. As of September 2013, there were 236.8 million active mobile phones and about 237.6 million people. The number of mobile phone subscriptions was at 285 million and rising as per the website - Living in Indonesia.