The species doing it
Through the older Shire streets — Sutherland, Kirrawee, Miranda — the usual offenders are liquidambar, camphor laurel, brush box and the occasional large fig. All were planted widely last century, all grow big, and all put substantial roots within the top 300mm of soil.
On shallow sandstone soils they have nowhere else to go. A root that cannot grow down grows sideways, and a path or a driveway crossing sitting on 75mm of sand is easy to lift.
What roots do to pipes
Roots do not usually break a sound pipe. They find a joint, a crack or a leak that is already there, and then they exploit it — old earthenware sewer with rubber-ring or mortar joints is the classic case, and plenty of it is still in the ground across the Shire.
So a blocked sewer with roots in it is two problems: the roots, and the pipe defect that let them in. Cutting the roots buys time. Relining or replacing the section is the fix.
What can be cut
There is no exact rule, but the working principles are these. Roots get more structurally important the closer they are to the trunk. Cutting on one side only is worse than spreading the loss, because it removes anchorage asymmetrically. And a large tree that loses several structural roots on the windward side is a different tree afterwards, even though it looks identical for two years.
A rough guide used in practice is to stay outside a radius of several times the trunk diameter when cutting anything substantial, and to get advice before cutting a root more than about 50mm across on a tree you intend to keep. If a council application or a build is involved, that assessment belongs to a consulting arborist.
Options short of removal
Root pruning plus a root barrier. Cut at a sensible distance, install a barrier, and reinstate the paving. Works where the tree is worth keeping and the conflict is local.
Reinstate the surface differently. Flexible paving, a floating deck over the roots, or a path realigned around them. Often cheaper than fighting the tree and it lasts longer.
Relining the pipe. Deals with the sewer problem without touching the tree at all.
Removal. Where the tree is a camphor laurel or a liquidambar in the wrong place, and the conflict will keep recurring, removal and a better replacement is frequently the sensible economic answer. Removal also means dealing with the roots left behind, which is covered in the stump options article.
Street trees are not yours
If the tree lifting your driveway crossing is on the nature strip, it belongs to Sutherland Shire Council. Report it. Cutting roots on a council tree without approval is an offence, and it is also how a claim against you starts if the tree later fails.
For advice on a specific conflict, call (02) 9538 7974 or see the stump grinding page.