Since the press never reports anything with enough contextual information, this “fuel mayday” means that the plane when landing was expected to have less than 30 minutes of reserve fuel. This would be at a cruising altitude of 1500 feet, though I am not sure if it’s different because they were landing in Australia (I know nothing about Australian flight regs).

Almost no plane has an actual reserve tank. That’s really not a thing. It’s just a calculation based on expected route, weather conditions, altitude, and cruising speed.

For understandable reasons, airlines don’t like to fly around with a lot of extra fuel as it’s heavy and reduces profits and is more dangerous on landing (plane more likely to go kablooey if something goes wrong).