Stick to the forum unless its properly private (this way others get to benefit from reading my wisdom you see )
You said, you need to connect up a flat 80m out the back of the main residence.
As far as I know, the long standing proticol has been one property = one internet connection. Even where people subdivided and built a second house, often Telstra would not connect up to the second property.
However what you need to do should be pretty straight forward. Ethernet (CAT6) can run for 100m. So just buy a cable and run it from the modem in your house out to a switch (optional) in the flat. Just beef up your NBN plan to cover two users using the same plan and split the bill.
Note check the cable you buy already has RJ45 connectors on each end.
PS Doing it cheap you can just drape the cable between the houses. Second option is dig a small shallow trench and lie the cable in it. Or do it properly with some electrical conduit
A switch is totally plug and play. It has no settings.
The switch is powered by a power point nearby, not the CAT6 ethernet cable.
Without knowing your exact situation you
1) You have an NBN modem in your house.
2) It is connected to a router/switch with many CAT6 plugs
3) You can plug the 100m cable into one of those plugs
4) At the other end you can plug the 100M cable into the switch,
5) The switch has many cat6 plugs
6) The person in the flat can use any of those plugs (plug and play)
A ... switch I provided a link to one above.
Ultimately your main issue is just laying a 100m cat6 ethernet cable from your house back to the flat. The cable is the hard part. plugging in a switch etc. is just something you do later.