Any surplus power that you generate that is not used in your household or stored in a battery will be sold back to the grid resulting in credits being allocated to your account. You can double check this information on the second page of your bill within the "Credits" section.
Thanks for the response but that did quite answer my question.
When I generate surplus power in the middle of the day that AGL sells back to the grid, I realise that AGL payment me a pre-agreed tariff however what happens when the actual price is less than this tariff or even negative?
AGL presumably won't be selling power at negative prices.
I saw that some of the commercial scale renewable companies actually get switched off (which protects them from selling power at negative prices.