We actually saw a Nationals candidate's poster in Tanunda - didn't know that the Nationals even fielded candidates in SA, but I guess Barker is a rural seat...
The Nationals were running pretty visibly in Mackillop (basically those bits of the Southeast that aren't Mount Gambier) in the state election when I was there in March, although I think they only got 5% or so. They did hold a state seat in the Riverland for a while.
Sorta makes sense then, as those state electorates are all covered by the federal electorate of Barker - although it seems unlikely that the Nationals will unsettle the party which has held it basically since its inception (to paraphrase the results of my Googling on the subject matter).