The most likely suspect was a bad starter/alternator cable. After some years, corrosion would set in. not a problem when the cable was cold after the car was started, but after the temperature increases, the corroded cables' resistance would increase, meaning less charge to the battery.
So here's how I tackled it. Usual disclaimers apply - I'm not a mechanic, all photos and descriptions provided are for amusement value only, any damage to your car or yourself is your own damn fault.
- New starter-alternator Y cable. About AUD$180 delivered from USA, as no local sellers had it.
- New drive belt. About AUD$40 on eBay. Cheap to replace as general maintenance.
- New alternator regulator - about $100 on eBay. Possibly not required, but seeing as I would have to get the alternator out anyway, a useful part to replace