There is an existing bike path from Marsh Street along the sewer which crosses the sewer and then passes under the M5. From the crossing point, the trail is to continue on the sewer and, after the junction with the sewer from Eve Street but before the pumping station cross over onto the sewer from Eve Street. From there it is to continue until a little bit before the point where the water pipe from the desalination plant goes over the sewer.
A bike path on a concrete sewer is definitely doable. An example is the sewer which crosses the Cooks River next to the Princes Highway. The sewer through Cooks Cove has some minor obstacles like small pipes going over it which may require some ramps over the top. There is also a pumping station. But by moving a small storage structure, enough space can be made to route a bike path past the pumping station.
Just to illustrate that a bike path on a sewer is not a crazy idea but something that has been done before.
Below is a picture of where the existing path crosses the sewer between Marsh Street and the M5 underpass. That would be the start of the new bike path which would continue along the sewer, behind the rider on the picture.
Below is a picture of the sewer line, taken from the existing bike path from Marsh Street. Some resurfacing would have to take place. This might well be necessary anyway as part of maintenance.
Below are some pictures of minor obstacles. They could probably be overcome with ramps. These are the biggest obstacles, apart from the pumping station. There are some smaller protrusions elsewhere along the sewer, comparable to the kerbs that separate foot paths from roads.
Below is a picture of the sewer from Eve Street joining the sewer from Marsh Street. The path is to cross over to the sewer from Eve Street after the concrete protrusions and before the pumping station which is visible in the background.
Below is a picture of the pumping station, taken from the east.
Below is a picture of the most significant obstacle within the pumping station, a huge pipe. There is enough space to route the bike path past it. That would be the most narrow point.
Below is a picture of a storage structure. It should be possible to move this structure.
Just for comparison, below is a picture of the cycle path along the Cooks River going under Canterbury Road, Canterbury. It's much narrower than the sewer path would be at the pumping station. But in my experience, cyclists and pedestrians manage even that tight spot at Canterbury Road without any issue and wait patiently if someone entered the underpass from the opposite side first.
Below is a picture of the pipe for desalinated water. The path is to come off the sewer before that and go south (right in the picture). Some sort of ramp would be necessary.
Below is a picture taken from the east, looking west over the sewer. To the left is the M5 and Cooks Cove South. To the right is the golf course.