I would not assume anything about the battery. It can't hurt to get it checked. Given that it is happening at low speed, perhaps your stator is faulty and failing to charge the battery. This would also be worse on slow technical trails or in traffic with a lot of starts and stops. Did you have any additional load on the system (heated gear, aux lights)? (Edit - I just saw you're in NZ, so I assume heated gear doesn't apply right now).
But I would agree, it sounds electrical, and the ignition switch wiring was probably a good place to start.