There are heating solutions for every application.  The three main heater types in Australia are solar, gas and electric.  

Solar
Solar heating relies on the heat from the sun and is generally only effective during late spring and early autumn.  It is a highly effective and efficient method of heating your pool during the warmer months, allowing the pool to be heated to in excess of 30 degrees when the weather is warm.  However the downside of solar is that long run times are necessary during the day in order to absorb sufficient heat, also during cloudy or wet periods your pool will not heat at all.  

Gas Heating
Gas heating is the most versatile method of heating your pool or spa.  Rapid heat up times and ease of use allow  you to quickly heat your pool or spa on demand, regardless of environmental factors.  When coupled with an automation system such as Viron Connect you can remotely turn on your gas heater and heat your spa up for when you arrive home from work. 
The only downside of gas is expensive running costs however these can be offset with energy saving pumps and running during off peak.

Electric Heating
Electric heating when done correctly is a cheap and effective moethod of heating a pool year round if required. An electric heat pump is basically an airconditioner for your pool, they take longer to heat a pool than gas however can be run off peak and coupled with a variable speed DC pump to counter the heater running costs.