They CASA)are responsible for VH registered aircraft wherever they are.
Kelpie,
Not quite true, I am afraid. the Civil Aviation Act 1988 makes provision for Australia to sign an 83bis agreement, in this case meaning that VH- aircraft could be based in and operate under CAAS rules ------ if there is an 83bis agreement in place between AU and SIN to cover that aircraft.
This is a common ICAO based arrangement.
Equally, validating non- AU pilot licenses to crew the aeroplanes (whether in AU or SIN) represents no obstacle.
Tootle pip!!