Thanks Kellie - I have found John ./ Johann in SA records. He died in Penwortham (SA) 1898 some time after prospecting for gold in the NT.
Christoph Wilhelm Krueger/kruger who was the older brother was born 1841 in Posen - this guy is quite elusive. There was a WC Krueger who died in Roebourne in WA but the ages do not correspond and I tracked back to his naturalization record but doesn't correspond either.