power.swan

I wish I had a dollar for every time I've posted this image.


In your case, setting 02 would be 100 A.
 

That sounds like your meter is totally out of calibration, malfunctioning. As mentioned by other posters keep a daily reading record and request from the utility provider to check the meter. In our municipality this can be requested. However if no malfunction is detected, you pay for the service. The crux is that there is no independent authority to check and calibrate the meters.
I ran once myself a check of the meter and found that it meters correctly within my measuring accuracy.

Arrghhh! Stupid me. He said "I have a whole lot of panels". My bad!

So, is the system set to export? Is the system registered with COCT and thus is "talking" to a bi-directional meter? Is the system able to stop export?

Again reading the meter is a useful exercise. If your own meter readings align with the City's then either the meter is badly calibrated or you have a lot more than you thought going through that meter (I didn't say your house is using more than you thought).

Can we perhaps agree that the Axpert/Mecer the OP has cannot export as a few members have mentioned already. This then rules out the possibility of exporting a lot and being billed for it.
Without more information I think it remains pure speculation with all the possibilities.
@Beat
You can get an independent company with calibrated equipment that can fight the readings in a court of law. It just cost an arm and a leg. If you find you have been over billed due to faulty meters and the utility has overcharged you for very long and it comes to R1mil or more it is worth it.

I was about to say this. You are probably feeding back and been charged as if it was usage. 

Some meters will bill you for all power regardless of the direction. If you feed in it will register as normal power usage (instead of export).
It sounds to me that you are possibly exporting to the grid during the day getting billed for each KWh. Then you are still consuming 8KWh and paying for that too.
Look at you meter closely and take regular readings to see what is happening.

Some major fault. If you are using only 8kwh per day from the grid your account cannot even be R1000 for the units used let alone R5000.
Need to look at the readings and rate of change of units in your CoCT power meter.

A few things 
Don't use inverter data to track usage, rather take readings in the morning and the afternoon at the main meter ( the new ones are all digital).
Check your inverter is actually powering the loads during the day from PV and Battery, and at night you should allow batteries to feed the house at some point, weather that starts at 6pm or 8pm is up to how much you consume.
You say you averaging 8kw from grid meaning your loads on the inverter are pulling 8kw from the inverter I suspect this could be at night ? I would assume so...... That 8kw is for the loads on the inverter and not the geyser and stove which the inverter cannot track as these are not hybrid grid tied inverters like the sunsynk so the inverter monitoring will have no insight on the geysers and stove .
Essentially take daily readings from the main meter and subtract 8kw from that, the remainder of that calculation should essentially be your geyser and stove.
Edit*** Geyser and Stove