In the NWFL in Tasmania in 2021 Devonport played Penguin for the premiership.There's an interesting similar case in the WAFL. In 1978, East Perth and Perth met in the WAFL Grand Final. This wasn't unusual at the time, East Perth and Perth had played in the 1966, 1967, 1968 and 1976 Grand Finals, the Perth Demons winning all of these, but the East Perth Royals won a thriller over Perth in teeming rain in 1978.
East Perth would have been confident of winning some more premierships in coming years, while Perth having won the WAFL in 1976 and 1977 would have been aiming to make it three flags in four years by taking out the 1979 WAFL Premiership.
However, it would take 18 long years before either of these teams would make a Grand Final again, East Perth losing the 1996 decider to Claremont, before the Royals would break their 22-year premiership drought in 2000 and would win the 2001 and 2002 flags too. Perth have not made a Grand Final in 45 years since then, have finished at or near the bottom of the ladder on many occasions, and their finals appearances have been few and far between, in 1986, 1991, 1997 and the COVID affected 2020 season.
I couldn't find any instances of this in the SANFL or VFL/VFA - in most Grand Finals where a less successful team has made it there has always been a powerhouse like Port Adelaide, Norwood or Glenelg in SA; or a Port Melbourne, Williamstown or Sandringham in Victoria.
In the Talent League the Northern Knights played the NSW-ACT Rams in the 1996 Grand Final. The Knights were going for - and won - their fourth premiership in a row, while the Rams did well to make the GF from 7th spot in their first season and were far from disgraced. Twenty seven years later, the Northern Knights have not made another Grand Final, while the Rams mainly struggled throughout the rest of their tenure as a full-time Under 18's team in the late 1990s and early 2000s, before they withdrew from full-time competition at the end of 2002 and from there on played 4 games a season on an invitational basis.
At the time Devonport hadn't won a flag for 33 years since winning the 1988 TFL Statewide League premiership and Penguin had gone 36 years without a flag, having previously won the 1985 NWFU premiership over Smithton in the days where the old three regional competitions were still operating prior to the TFL going statewide.
Currently in the Southern Football League in Tasmania, there are three clubs with long premiership droughts, Sorell last won a flag in 1990, Brighton's last one was 1998 and former TFL club Hobart hasn't won one since 1999.