Rebel1, was a Smartmeter recently installed at the property ? And are you on a Time Of Use(TOU) tariff ?
If so, then it is most likely a billing issue with the energy retailer's computer system not being able to interpret the "NEM12 data" correctly. It is highly unlikely that the Smartmeter is at fault.
"NEM12 data" is the information that is stored inside the Smartmeter, which is transmitted to the Australian Energy Market Operator(AMEO), then, in turn, is sent on to your energy retailer.
You asked "How do we know that the amount of electricity we are told we use...we actually use???"
By taking readings from the meter at given intervals anyone can easily see what their energy consumption is. No need to buy third party "plug in gadgets" to measure energy consumption. Without taking readings you do not have a basis for any argument with your retailer over billing issues relating to consumption.
Alternatively you may request from the network operator to have that NEM12 data sent to you so you can check it against your account. Though, some operators may charge a fee for the service.
But from what you have posted, there is not much to go on, you need to give a lot more detail about your meter, the tariff and what is stated on the account summary. Cheers!
