Deja Vu indeed!
Same thing happened to me a few weeks ago. The battery was old and wouldn't take a charge, got it replaced but that drained overnight. Called out the NRMA and he said the battery was okay but the alternator looked shot.
To avoid chasing everything, replaced the alternator, regulator, and starter solenoid together - she starts better than ever now and hold a charge. (Though I now keep it on a trickle charger just to keep the battery humming along.)
So, as Dwayne suggested, get the NRMA out to check it.