Short answer
A National Meter Identifier or NMI, is a unique 10 or 11 digit number given to every networked electrical connection in Australia.
Where you can find it
On your bill
You will find your NMI on the first page of your electricity bill under Account details.
In My Account or the Origin app
Log into My Account or the Origin app
On the Dashboard in your electricity account, select the Usage link
In the Usage screen, scroll down until you see About your electricity meter, then
select +. You'll see your NMI when you expand the screen.