Doing this is easy as you only need to buy a 100x90mm stormwater adapter and a 100 x 50mm DWV invert taper.
1. Buy and fit a meshed 90mm overflow outlet.
2. Buy and fit a 90mm pvc stormwater 90 degree M + F elbow. Fit the M end into the meshed outlet's socket.
3. Fit a short 90mm pvc stormwater pipe (sleeve) between the elbow's female end and the stormwater adapter's female end.
4. Fit the invert taper's largest end over the stormwater adapter.
5. Fit a 50mm DWV pipe to the invert taper.
The DWV pipe is much stronger than a 90mm pvc stormwater pipe.and it won't have any problems with the flow due to the vertical riser providing head pressure should water in the pipe ever back up.
Thank you - that's super helpful.
Should I use this reducer:
https://www.bunnings.com.au/vinidex-90mm-stormwater-50mm-dwv-adaptor_p4770401
Or this one?
https://www.bunnings.com.au/holman-90-x-75-x-65-x-50-x-40mm-storm-pvc-stepped-adaptor_p4760090
If we left the 50mm DWV pipe on the ground surface (instead of burying it), would we be able to go over it with a ride on mower (without breaking it)? We could bury the pipe just under ground level, but then the end of the overflow pipe (which will be going in a hedge) will be buried too, and it will block.
Also, this is a little off topic, but with our 2 x 22,500L tanks, we placed sleepers around them and then removed the earth, filled it with crushed rock then crusher dust (about 200mm) and compacted it all. We thought we had done a great job (well, a plumber did it not us), until it rained. Every time it rains heavy, the sleeper box around our tanks fills with water (and the water takes forever to drain). The main issue with this is that our tanks are adjacent to our shed, so the water slowly seeps under our shed. Is using sleepers around water tanks a bad idea? I thought that's what people do to keep the tank foundation from washing away.