Obviously you did not read the website before the trip to realize the weight system applied. However, you might have a case based on their poor website...

I read both the English and Chinese versions:

CSN will issue an overweight baggage ticket for overweight baggage. The overweight baggage will be charged based on 1.5% of economy class airfare on the day when overweight baggage ticket is issued. The charge is calculated in CNY, with the mantissa rounding.

旅客的托运行李和自理行李，超过该旅客免费行李额的部分，称为逾重行李，应当支付逾重行李费。

收取逾重行李费，应填开逾重行李票。

逾重行李费率为每公斤按逾重行李票填开当日所适用的经济舱票价的 1.5%计算，以人民币元为单位，尾数四舍五入。

The English version did not include the key words - per Kilo!

The website is confusing. The tables talk about the US/Canada routes piece system and the European routes. It does not talk about Japan-China-Australia. So, I am guessing the calculation is based on the economy fare as quoted above and I believe the convention is the full Y economy fare, not the discounted fare.

Cathay, for example, has a very clear website on EB (Excess Baggage) charges and does not go by fare, which is unusual. Japan-Australia would be US$40/kilo x 28 kilos = US$1,120. http://www.cathaypacific.com/cpa/en_...eckedbaggage#Z Similar for SQ: http://www.singaporeair.com/saa/en_U...gage_zonal.jsp

Good luck. If they are smart, there might be some disclaimer on the website. If not, you might argue this with your credit card company and get the money back.

Since this is history, I think "how much was carried by whom" can't be traced other than based on the receipt(s) you have. If they lumped them together into 1 passenger, then it might help your case. (Then of course the poor agent who did you a favor might get penalized.)

I worked briefly at an airport in Asia for an Asian carrier. The SOP for passengers on EB: beg first then get mad. The SOP for agents: charge first then discount and compromise.