Would be very silly as would have a high chance of being damaged…being on the footpath/accessible side of the fence would be an invitation to vandals.
I suspect that they have the powers vested to them from Commonwealth legislation (and corresponding State and local government legislation) that allows them to locate the NBN infrastructure in public lands/common lands/road reserves, in a manner which suits their purpose and doesn’t impact on other infrastructure. Possibly a fence would achieve this but wouldn’t be very secure or a long term solution.
For units, I can see why they would run one line across the unit building to service all units. This would save running separate lines to each unit, some of which could potentially have a common route. Bundling them together also makes them easier to find when work is carried out on the buildings/grounds (and using a service like dial-before-you-dig).