The way I see it is on DJ and JQ (im not sure about qantas) here in OZ the carry on bags should not exceed 7 kg and this is clearly outlined at the airport and check in as well as the baggage rules when buying a ticket. I would be fine with lifting up to 7kg but not over. If it was heavier i would politely explain that they are not meant to carry on baggage that weighs more than 7 kg, so i can arrange to put it under the plane for them or they are going to have to find someone else (perhaps another strong PAX who can lift it) unless as others have said they are small/frail/old/young/disabled etc. Customer service is important but so is our own safety.
