Where will they obtain an additional steam locomotive from? A new build perhaps?

$16 million upgrade to Tasmanian Railway

According to this website http://www.australiansteam.com/tasprivateframe.htm there's one at the Tasmanian Transport Museum. Confirmed by https://tasmaniantransportmuseum.com.au/exhibits/rail-exhibits/locomotives/ Perhaps giving up their Abt might be a decent swap for being able to run out on the mainline again.

According to this website http://www.australiansteam.com/tasprivateframe.htm there's one at the Tasmanian Transport Museum. Confirmed by https://tasmaniantransportmuseum.com.au/exhibits/rail-exhibits/locomotives/ Perhaps giving up their Abt might be a decent swap for being able to run out on the mainline again.

Being the last of 4, you would think its in the worst condition, but with enough money anything is possible.

They were able to pull ex ABt Railway assets away from other users that originally said they were not for sale in the past, so anything is possible for the right price. Physical access to the mainline and resolving the ongoing issues of insurance etc that has blocked T&HR operators from Tasrail for more than decade is likely to be part of it.

From your link, they obviously know about it.

ABT No.2 was obtained by the museum as static exhibit and is currently the only engine in original 1960s condition. ABT No.1, No.3 and No.5 have been rebuilt for use on the West Coast Wilderness Railway and are once again hauling trains between Queenstown and Strahan. A current proposal may see ABT No.2 restored to working order for use on the West Coast Wilderness Railway

there's also the one that's sunk in a quarry

http://www.australiansteam.com/4%20Abt.htm

and theres the non rack Baldwin there too