Historically the original Main South Line through the Dunedin suburban area was built as a single track. Between 1908 and 1948 it was rebuilt between Mosgiel and Sawyers Bay, this included doubling, and up until now I always believed that the entire length between these two points was doubled, 26 km in all.
However, the NZ Railway & Tramway Atlas 4th Edition, which is the current edition in print, shows that there was a single track through the Sawyers Bay tunnel and the causeway immediately to the south of it. The drawing style different from that used in other areas where an original double track was later singled. The way it is drawn suggests the line has never been more than a single track in the area.
I have recently been in Dunedin and the tunnel was clearly constructed as a double track bore and there are presently two tracks through it. The double track from Sawyers Bay south currently ends on the causeway itself a short distance from the tunnel at the arrival signal SRB 4R. The current two tracks through the tunnel appear to be set out as the main line and loop at Sawyers Bay station. I don’t have enough info about the area to know whether Sawyers Bay is in SLA or CTC signalling area.
The rest of the causeway appears wide enough to support two tracks and the bridge abutments were made wide enough for double track. I guess the real question is whether double tracks were ever laid beyond the 4R signal at Sawyers Bay. In 1948 when this last causeway was completed and opened, there were a lot of competing demands and resources were scarce, it is somewhat possible that the line could have been opened as a single track with the second track unfinished.
So this is an area where more research would be needed to confirm whether this anomaly is supported in actual fact.