Run-off is usually quoted as an annual percentage, and it can be measured two ways: by loan count (how many of last year's loans are gone) or by balance (how much of last year's trail-earning balance is gone). Balance-based run-off is the one buyers care about, because trail is paid on balance, and it captures both loans that left and loans that were paid down fast.
Commonly cited figures for Australian residential books sit between 15% and 25% a year, and spike after rate cycles when fixed periods end together. Because run-off is applied to a shrinking book and the loans that left never return, its effect compounds: a 20% run-off removes about a third of trail in two years and about two thirds in five.
Run-off is the number most within a broker's control. It falls when clients are reviewed before their fixed rate expires, repriced before they ask, and contacted when their balance drops sharply, all of which are signals a commission file contains if someone reads it every month.