Terrace gutters fail differently to house gutters. In Marrickville a great deal of the guttering is box gutter running between two roofs and draining through a single outlet, often shared with a neighbour. When that blocks there is nowhere for the water to spill, so it goes under the tiles and into a ceiling instead.

Marrickville's Victorian and Federation terraces and semis were built to the boundary, and that determines almost everything about the guttering.
Instead of a simple eaves gutter running to an open edge, a terrace typically has a box gutter set between its roof and its neighbour's, draining through one or two outlets. There is no safe failure mode in that design. A blocked outlet ponds water on the roof, and the weakest point is usually the valley lining or the flashing above someone's back bedroom. Because the gutter is shared, the blockage is frequently not on the side of the property that ends up wet.
The debris is a mix. Plane trees and figs along the main roads and around the Cooks River drop heavily, and the fine particulate that settles on this suburb from the flight path and the road corridors binds that leaf into a dense, slightly greasy mat that ordinary flushing will not move.
Rear lanes and zero-lot-line boundaries make ladder access genuinely awkward, and a ladder footed in a narrow lane or against a century-old parapet is not a safe arrangement.
Vacuum clearing from ground level handles the reach, pulls the packed material out rather than pushing it toward the outlet, and we check outlet flow specifically because that is where the expensive failures start.
The strongest reason to keep a Marrickville terrace gutter clear is that the consequences of not doing so are rarely yours alone to control.
A box gutter shared with the property next door means a neighbour's neglected outlet can put water through your ceiling, and yours can do the same to theirs. Once that happens you are into insurance, make-good and a conversation about who was responsible for maintaining a gutter that sits half on each title. Clearing on a schedule, and being able to say when it was last done, is worth a great deal in that conversation.
The building fabric adds to the case. These roofs are a century old, and the valley irons and box gutter linings under all that wet leaf are often near the end of their service life already. Sitting them under a permanent damp load accelerates corrosion, and relining a box gutter on a heritage terrace is roofing work rather than maintenance.
There is a stormwater angle too. Parts of Marrickville drain toward the Cooks River and flood readily, and overflow that should be going down a downpipe instead sheets against the base of a wall on a property with little or no setback.
Ground-level vacuum work avoids ladders in narrow lanes entirely.
How vacuum gutter clearing works, what we check while we are there, and what it costs across Sydney.