The buses, to be provided by Scania Australia in conjunction with Custom Coaches of Adelaide, will replace the existing fleet of non-articulated Renault diesels, some of which are now 25 years old.

Mr Rattenbury said the new buses, which will meet Euro 6 emissions standards, also have a design life of 25 years, meaning some could still be in service in 2042.

No consideration has been given to possible future legislation requiring the fitting of seat belts.

''Fitting seatbelts to ACTION buses would result in the carrying capacity being significantly less as the bus would no longer be able, under law, to carry standing passengers,'' a TAMS spokeswoman said. ''The new buses carry 48 seated and 18 standing.''

The existing Renaults can carry 43 passengers seated and 27 standing.

The funds for the new buses were set aside in the 2012-2013 budget.

Like the buses they replace, the new Scania fleet will be fitted with bike racks capable of carrying up to two bicycles.

While they have significantly lower exhaust emissions, the new vehicles will not use less fuel. ''The new buses have much cleaner and more fuel efficient engines but are heavier than the old Renault buses due to having extra safety features and being inclusive of air conditioning,'' the spokeswoman said.

''Due to the extra weight the buses are expected to use more fuel [than the Renaults] but will produce less emissions and will provide bus riders with a more comfortable travelling experience.''

The switch to a more modern bus fleet is not expected to lead to job cuts at the ACTION service centre.

''The new buses have a lot more systems on board and [are] air conditioned and this may require additional maintenance,'' TAMS said.

The 12.5-metre rigid Scanias will join the recently acquired fleet of 13 articulated buses that have recently been delivered to ACTION.