Stedman Genealogy http://johnlisle.us/genealogy/getperson.php?personID=I5436&tree=stedman_uk says he died aged 22 in Western Australia around 1896, possibly speared by an aborigine at Yalgoo. I serched trove but found nothing to this effect, nor anything to support him being a police officer in W.A. or dying in Western Australia. A death of this nature would have been subject of a court case.
Findmypast.com.au has a Wm Turner death in 1898. in W.A.
It may be possible to confirm family relationships with William by comparing test results with other carriers of his ancestors' Y-chromosome or mitochondrial DNA.
However, there are no known yDNA or mtDNA test-takers in his direct paternal or maternal line.
It is likely that these autosomal DNA test-takers will share DNA with William: