Bus passengers in Sydney are so heavy that the Federal Government is being asked to review the loading capacity of buses, which is currently set at 65kg per person.
In fact, health experts have said, if taken practically, the average weight should rather be taken around 72kg, given most buses only contain adults.
The Government said it was reviewing national bus standards but not specifically passenger weights.
The spokesman added: "A consideration of the notional mass of a passenger for the purpose of calculating the occupant capacity of buses was not part of the review . . . there is no proposal to amend the notional mass per person."
The Rail, Train and Bus Union said that they would have to update the weight per person capacity.
"Obviously the 65kg figure is out of date and it needs to be revisited to more realistically reflect today's population. (If the number changes) the number of allowable passengers that is currently displayed will have to be reduced to reflect that," said RTBU divisional secretary Raul Baonza.
The Roads and Traffic Authority said the matter was in the Federal Government's hands.
"There is a national standard, set by the Federal Government, which is used when calculating the loading capacity of buses. This is (currently set at) 65kg per passenger," said an RTA spokesman.
Health professionals claimed that the average weight of adults over 25 is now 78kg, with men aged 50 averaging 85kg and women aged 50 coming in at about 72kg.
While the current figure of 65kg includes children, but unions claim that thousands of peak-hour buses every day travel without school children. On atypical work day, Sydney buses make 15,000 trips carrying 600,000 passengers.
Almost, 100 "bendy" buses in Sydney are licensed to carry 88 passengers, while the normal-sized buses are authorised to carry 62 passengers.