I have a battery and joined the VPP. In the evenings AGL draws power from my battery, sometimes up to 20% of its capacity. OK, that's what a VPP is for.
However, AGL pays me 12c per kWh and then I have to draw power from the grid during the night (because the battery is exhausted) at 22c per kWh. So AGL is effectively charging me 10c a kWh to help provide power at peak times, and on top of that they are no doubt selling that power to other consumers for more than the 12c they paid me, so they profit twice.
AGL should offset any power drawn from the grid against power taken from a battery, i.e. they should cancel each other out, at least over the next 24 hours.
It is easy to tell what power comes from my battery versus solar -- the battery power is after the sun goes down. In any case, the AGL network has to send a command to the battery to start/stop exporting power.
You can always turn off the 'export stored Powerwall energy' function in the Tesla App and just have PV export to grid (or none is the other option).
I have a 2cent difference in price from off-peak to FiT until 1/8/22 when the difference jumps to 5 cents.
Usually I can get through the night without emptying the Powerwall 2.