My mate bought a BA MkI about 4-5 months ago and everything was fine.

He usually drives the fuel to nearly empty ie 20-30km left on hte range. He has never run out of fuel.

The other day we were driving when when range said 86km left and the fuel gague had about 1/8th left on display when we ran out of fuel. As we thought it was a one off we didnt do anything (except fill up) but when he filled up with 100km left on the range and 1/6th on the fuel gauge and it took 66L.

Any ideas on how to fix?

I thought maybe unplugging the battery to reset the ECU, maybe that would do something.

hey mate, its pretty common on the BAs.
The estimate k's remaining is worked out on the fuel consumption, so if you cruise for most of the tank it's going to think you're using 12l/100km or whatever, but if you start thrashing it at the end of the tank it's going to use more fuel, but wont be able to properly account for it.
best way is to drive the car the same way all the time.
I always filled the car at 100km to go just to prevent this problem. it's easier than having to use a jerry can.
the 80km bell is supposed to mean that you have 10ltrs of fuel remaining, but i had the car stop at 87km to go once.

on a completely different topic, i had a subaru legacy GT do 200km after it reached the empty mark when i was driving back from geraldton once..

its a real shame the BA sensors aren't like cars of old, that have a little in reserve just in case..

i hope this helps.