In Australia, there are only 3 mobile networks. However, there are many resellers (companies who provide mobile services). The 3 networks are owned by Telstra, Optus & Vodafone. They are all good and have good coverage in the capital cities. 

Most resellers have prepaid and postpaid price plans. Some of them provide an option for including a mobile device as part of the plan. The price plans provide different pricing options and services. Some providers include international calls to only a few countries in their price plans. 

There are many websites, which provide a free comparison for the available plans from different providers. However, not all of these websites include ALL mobile service providers. They might do a comparison for only a subset of the mobile service providers.

Finally, please note that you can change service providers without changing your mobile number. If you are not sure which plan suits your needs best, make sure you do NOT commit to a 12/24 month contract with a service provider. So, you can change the service provider easily, should you want to.

When it comes to the coverage, as far as I read and found true Telstra has the best coverage then Optus and finally Vodafone. As Bilal said in the capital cities, you wouldn't notice that big difference in coverage in most of the time. When you want to choose the best mobile plan, it depends on your needs. A good website to help you compare many of them but not all of them is whistleout.com.au. from my own experience, if you need lots of Data then Vaya is the cheapest. If you need international calls compare Lyca( one of its bundles gives you unlimited calls to almost 60 countries excluding Egypt), ALDI, Lebara, iiNet, jeenee( 300 dollars international calls to any country), dodo ( 300 minutes to Egypt) and Amaysim ( unlimited call and text to 10 countries). Take into your consideration that some of them cover a few countries in their plans like dodo and aldi and others cover all countries like jeenee. To sum up, it's a trade off between price,data, international calls, coverage, customer service, roaming and coverage. 

I use TPG, which is a reseller for vodafone. a 19.99$ give me 550$ cap (all can be used for  international call) and 1500 mb data.

 TPG will install its physical network starting from the next year after acquiring mobile bandwidth in the last auction.

iinet also provides international calls for all countries for plan $40 includes 300 international calls.

I have noticed that many mobile service providers started to include international calls in their plans. The following link provides a list of these plans.

https://www.whistleout.com.au/MobilePhones/Search?calls=10&sms=50&data=2000&simonly=true&customer=Personal&currentsupplier=TPG&includefeatured=false&address=Bankstown+NSW+2200&internationalcalls=true&tab=postpaid 

If the link above stops working, I suggest you visit the website's home page below. Then, add a filter for the inclusion of international calls.

https://www.whistleout.com.au/MobilePhones