I recently (2 months ago)) bought a 1990 Holden Commodore VN to drive around Australia. After 10,000km (Melbourne to Cooktown then to Alice Springs via Tennant Creek), the battery warning light sometimes switches on. The car starts first time, every time, and my friend’s battery charger/tester tells me the battery is in good condition. Any ideas.???

The battery light measures your charging system, ie. your alternator. Likely the alternator output is weak.