I've not used these exactly but something similar. Though speced at 250V 12 to 16A I would consider these as low signal devices. I'm not quite sure how you define ammo box but these should work all right at 12V. I would not shove much past 6 to 8 A through them. The wire clamps are small so AWG size cannot be very large and the screws being small as well can easily be damaged by over-tightening.
Regarding the 240V rating, unless specifically stated one must assume that voltage rating is Peak. Wall voltages are RMS and to convert to Peak one multiplies by 1.414. Thus the Peak Austrailian wall voltage is well over 300. A voltage beyond this devices rating. Working backwards, without better device specs, using this device with any voltage above 170VAC would be considered hazardous.