Sydney’s problem is humidity, and it does not switch off in winter. Air that is already carrying moisture cannot absorb much more, so a shower in a Sydney bathroom leaves the room wet for longer than the same shower in a drier city – and an underperforming fan has far less margin before mould appears.
The second factor is density. A huge share of Sydney bathrooms sit in apartments with no window at all, extracting through a shared riser that serves the whole stack. When that riser or its roof fan is neglected, every apartment above it loses extraction at once – which is why so many of our Sydney callouts end up being building problems, not unit problems. We service single apartments and whole blocks.