The line was opened as single track on more or less the same alignment on 1 November 1877.Lots of stakeholders involved, and pretty sure it's not the track owners responsibility.It was a "railway's" bridge so would be ARTC's problem now, unless there's a get out clause in their contract. However, I'm sure that RMS is a more than interested bystander.
Bridge ownership tends be by who needs/wants it built. The road would have pre-existed the railway. The current bridge may have replaced a single track structure, or maybe even a level crossing.
The line was duplicated on 17 June 1917.
I assume the bridge has the "call John Holland" signs on other bridges in this area.
But I don't imagine the designers expected it to last 103 years, given the previous one had lasted less than 40 years.
While it was a geological failure rather than structural, the greatly increased loading of trucks today would have been at least partly responsible.