Using past weather data, on can approximately calculate the kWh output a solar system can create. The Clean Energy Council also supplies guidelines. The internet also has websites which can show the output of different solar systems in different parts of Australia. Just look for solar system output calculator on the internet.
Some solar companies deliberately overestimate the solar system production in order to achieve a fast Return on Investment (ROI) number.
Have an authoritative website available e.g. via Clean Energy Regulator for end customers to check the likely solar output in