What is this structure across the Brisbane River?

These two photographs from the John Oxley Library's collection show a structure crossing the Brisbane River between Auchenflower and West End.

Is it the preliminary work for a bridge that never progressed to completion, or the laying of cross-river cables, or something else?

These images will be featured on an upcoming segment of Channel Seven's Great South East program, where the identity of this cross-river structure is briefly discussed.

If anyone knows what this structure is, we'd love to know, so please blog back.