We have been experiencing this over the past 3-4 months. Lots of fines because they are falling off.

We changed envelopes away from the brown recycled ones and that seemed to improve it a bit.

We changed to the larger profile self-adhesive stamps in the books of 200x (not the rolls).

These are much better but only if you moistem them first. Yes... they are supposed to self stick but a little water makes it better.

Aust Post will not acknowledge there is a problem, not at our local PO or over the phone. So now we are seeing this happen about 1 in 250 or so envelopes we send. We seem to have no choice but to just suck-it-up.