During Mr Scott's subsequent absence in England on official business, Thomas Middleton, Locomotive Engineer, had the specifications altered to 0-6-0T wheel arrangement, together with other variations to the original order.

Middleton claimed that the revised design would enable the locomotives to run at 30–33 mph (48–53 km/h) in suburban service.

Commissioner Goodchap approved the changes and despite the protests on Scott's return, the variations were made.

Their 4-foot-0-inch (1.219 m) driving wheels proved too small for the speed required and they were reduced to shunting duties in Sydney Yard, with just the occasional venture into the suburbs.

Later, with the cranes removed, they were restored to their Z18 numbers and worked at Port Kembla shunting.