Here are the past ones that spring to mind which come close (if not more than that in some cases):
- 2UE went down from 8.4% in S1/1986 to 4.7% in S1/1987.
- 2GB went down from 11% in S2/1988 to 6.9% in S2/1989.
- 2MMM went down from 19% in S8/1991 to 12.4% in S8/1992.
- 2SM went down from 7.1% in S8/1994 (as Gold 1269) to 3% (as KICK-AM).
- 2WS went down from 10.5% in S1/1998 to 7.1% in S1/1999.
- Mix 106.5 went down from 9.2% in S8/2000 to 5.7% in S8/2001.
- 2UE went down from 13.1% in S7/2001 to 8% in S7/2002. This was due to Alan Jones & Ray Hadley moving across from 2UE to 2GB.
- 2Day went down from 9.3% in S1/2013 to 4% in S1/2014. This was due to Kyle & Jackie O moving across from 2Day to KIIS.
As for the reverse:
- 2UE went up from 4.9% in S4/1987 to 11.9% in S4/1988.
- 2Day went up from 8.7% in S2/1990 to 15.7% in S2/1991.
- 2WS went up from 7.3% in S6/1992 (on AM) to 14.1% in S5/1993 (on FM).
- 2UW/Mix 106.5 went up from 4.1% in S1/1994 (as 2UW) to 7.2% in S1/1995 (as Mix 106.5).
- 2MMM went up from 8.2% in S2/1998 to 12.1% in S2/1999.
- 2GB went up from 3.3% in S8/2001 to 10.2% in S8/2002.
- 106.5 went up from 4.3% in S1/2013 (as Mix) to 8.4% in S1/2014 (as KIIS).