Living in Australia does not mean having to spend hundreds of dollars calling back to India to speak to your family. There are many ways to call India cheap. The best way of course is to use prepaid phone cards. But given the amount of competition, it is best to compare your options via sites such Phone Card Star to assess what the best calling card plan for you may be.
Considering the cost of the card: they are usually sold for $10, $20 up to $100 of value. Usually, it is better value going for the highest value as you get cheaper rates. But, many phone card retailers have made a good living by providing extremely cheap rates to niche countries such as India.

Good phone cards will allow you to have access to support services online and on the phone, in case you have difficulties with your card. Be wary of cards that do not offer this avenue.

You should also be able to make calls to anywhere in India not just Delhi, but Hyderabad, Maharashtra, Orissa and all the way down to Kerala with perfect clarity.
There are plenty of good calling cards out on the market. There are cards that let you  finish talking before the call is disconnected for insufficient funds. Others cards will sound a beep 1 minute before the deposit runs out, which gives you time to say goodbye until the next time.
Be warned of cards that only offer you say $5 credit limit and no more. You will soon find that mid way through your call to India, your line may become fuzzy, unclear and eventually drop out.
Make sure to Google calling card brands that specialise in calling India. Some good search queries include:
calling cards to india
telephone cards to india
call india from australia
Australian calling cards to india
calling to india from australia