Yep tree roots and sewer pipe great combo
Don't have willows in NQ so can't speak with personal experience on that species. But we have numerous others that just love that special environment to be found in black water.
Most of the newer poly pipes are good in the ground for 10-15yrs, its the older clay pipe or poorly installed unions in poly that allow many roots in, and they only need a minute crack much less than .1mm.
Barriers have been used all over the globe and from the reports I've read and the roots through barriers I've pulled out they not all that successful. If it were a tree of importance to the client and they were willing to spend $$ to retain it but wanted to aleviate the root problem then sure hydrocutting and installing a barrier would be a good starting option, but for me having the pipe trench properly dug lined and sealed with properly installed pipework is realy the answer, that and moving the services further away.
As with everything its only a temp measure that black water is just too good to miss for mister tree
SF