I found this thread interesting because of the discrepancies in the weights returned by two different scales.
In Western Australia, the issue is covered under the
Trade Measurement Act 2006. Most states would have similar acts and legislation covering this.
The act states:
a person uses a measuring instrument for trade if the person uses it, has it in possession for use, or makes it available for use, to make a measurement for the purpose of;
(b) determining the amount payable as a tax, rate, toll, duty, charge or other impost (however described).
By definition, because the entity (the airline) uses the scale for determining if the baggage is within the allowed limits - and charging fees should it exceed them, then the scale and use of would be covered by the act.
The act requires that any measuring device used for the purpose of trade must bear an inspectors mark;
10. Measuring instruments used for trade to be marked
(1) A person who uses a measuring instrument for trade commits an offence unless —
(a) the instrument bears an inspector’s mark or a licensee’s mark; and
(b) the inspector’s mark or a licensee’s mark continues to have effect in accordance with section 21(4) or (5), as the case may be.
Penalty:
(a) if the measuring instrument is a weighbridge — $10 000;
(b) in any other case — $5 000.
In the relevant scenario, If it was used for determining an excess baggage charge and it did not bear a current inspectors mark - then the airline would be committing an offence.
Further, if the device was found to be giving an incorrect measurement - not only would the owner of the weighing machine be liable, but the Check In Agent would also be personally liable;
14. Instruments that are incorrect or unjustly used
(1) A person who uses for trade a measuring instrument that is incorrect commits an offence.
Penalty: $20 000.
I can't say I've ever looked out for an inspection mark when having my bags weighed, but if I was ever going to be hit with an excess charge when travelling in Australia I'd be on the lookout for the inspection or valid licensee's mark, and if not there - challenging the charge.