The furthest behind a player has been who has ended up winning the Coleman Medal is 32 by Tony Lockett in 1991...
I have by no means gone through every season, so there may be other significant deficits other goal kickers have overcome.
21 Brian Taylor/Warwick Capper 1986 R8
17 Harry Vallence/Doug Strang 1931 R4
17 John Coleman/Jack O'Rourke 1951 R6