Air handlers and furnaces aren't often used side by side in Melbourne. If you use a furnace, you likely shouldn’t need to spend time thinking about an air handler.
Air handlers are typically paired with heat pumps and control airflow throughout your residence. Some systems also offer backup heating and cooling components to help the heat pump.
A furnace runs in a different manner by pushing heated air into your vents, which circulate it around your residence. Because furnaces have combustion chambers to create hot air, they don't have some of the components you'll find in a standard air handler.