The main conclusion so far I have reached by other means (such as by examining the photos in this post) is to suggest the bridge shown above was probably between the present bridge and the later rail bridge. It appeared to be more in line with a road that came down onto it and therefore possibly was aligned onto the highway without the S bend that there is presently on the northern approach. There are also suggestions of possibly old piles in the river at this point and an old abutment at the south side that tend to align with these suggestions.
As for when the railway bridge was separated out: well the research shows that a new rail-only bridge was under construction in 1903, and that it was commissioned some time around 1905. Just why it was desirable at such an early time as 1903 to have a separate bridge is somewhat hard to understand, since many longer combined bridges around the country, some with very heavy levels of both road and rail traffic (such as the ones at Rakaia and Waitaki south of Christchurch) waited until much longer to be separated. The best info I have come up with is from The Cyclopaedia Of New Zealand (published 1905) which said the new location was “more suitable for traffic purposes” and “escaped a large curve”. This means the rail traffic must have been reasonably significant at the time, considering how much such a long bridge would have cost to build. The present highway bridge was built around 1930 and NZTA recently spent $600,000 strengthening and earthquake proofing it, so the highway traffic must also have demanded a better bridge, which is still standing more than 80 years later. However, it is acknowledged that this bridge is narrow by modern standards and it is programmed to be replaced in the next 20 years, being considered to have high maintenance costs.
Here is a map showing how things might have worked, plus several more useful photos.
This photo by Shane McDowall.
I believe this photo is by Royce Flynn. The bridge was finally demolished in 2002.