@Peej you are getting that cost info from looking at app usage, daily 30 minute blocks, see bought from grid for a 30 minute span, then it's more for Xkwh than 8c ??
All seems fine with my app and this figure, and the hourly blocks on the website usage page.
In this example, you are taking the $ amount divided by the Kwh used ?
If you are taking your figures from the website account usage page, the night section at bottom, this also includes any CL (controlled load) usage, for example a HWS (hot water system) operating, as this does not use the EV rate for 'all power into the home' on the main circuit.
See 1 hour website example below, but we don't have the HWS on CL, it's on our main circuit now.
As you said this seems to have only changed after getting a 2nd EV, it could be something else, a CL charge would show up pre that change.
The 2c FIT ?
From July 1st tariffs and FITs changed as they usually seem to do, upwards and downwards respectively.
Our FIT in SA dropped from 2c to 1c !!
You should have had a letter or email from AGL advising changes to your plan, typically this happens Australia wide on or around July 1st each year.
I expect FIT might always be present in some form with retailers, perhaps eventually being a fraction of a cent, I think many retailers in various locations already pay under 1c.
But hopefully FITs will not go negative as you see with some wholesale and very limited retail plans.
There is one retailer in SA that offers a 30c FIT for 3 hours evenings (great for battery owners), and in solar soak a negative FIT of -2c, I think that is usual solar soak in SA from 1000-1500.
The ability for a system to curtail feed in is then vital for this scenario.