It's a pipe venting system designed to handle typical city sewerage over the course of a couple of years of observation prior to 1990 (when constructed).

   The August report reveals that FOG in the Malabar wastewater system have risen by 39% over the past 10 years. Volatile organic compounds – including cleaning products, cosmetics, paint, fuels and other chemicals – have risen by 125%.

What it was designed for has changed, leading to unanticipated clogging in unexpected locations .. there's now a mass back forced into an zone that was intended to be clear and that mass is fragmenting and feeding chunks back into the decline outflow.

Disclaimer, I've not read the detailed report, and my poor understanding is that Sydney water themselves is doing a bit of guesswork themselves here.

A big issue is it was considered most economical and acceptable in the later part of the 1980's to push barely treated solids in a slurry a "long way" out to sea rather than invest in a full treatment plant that deals with everything on land and emits drinkable water.

Addendum: To add to the problem Engineers (civil) faced, I'm now reasonably sure they in the 1990s were tapping into an out to sea decline fall from the 1930s.

Accountants would have stressed the cost savings of this, over ruling any Engineers muttering about for only four times the capital costs they could have something much better and more future proof (maybe).