There's been a number of different bridges across Newport South Wales station, and there's another one under construction at the moment.
At one stage, the internal bridge between the platforms also had an entrance / exit onto the road at the back of the station. This wasn't always staffed (there were no gates then), so I'm sure that people used to use it as an access to the station and take their chances if travelling. The other end, through the old station building, was mostly staffed, so if you tried to get out into the town, you'd probably be stopped - but what they would do about it, I don't know.
The bridge access to the rear entrance was later closed and removed; it was replaced with a ramp and a very manky tunnel underneath the station which surfaced outwith the station. It's this tunnel which is now being replaced by a new bridge which starts and ends outside the station and will not have any access to it, so there'll be no way of getting access to the station.
The main internal bridge via the new station building has a gate line at both ends, although at certain times it's not staffed, so can theoretically be used for access to the town, but you're running the risk of one end being staffed even if the other one isn't!