Carl Johnson in Australia

Hi all, I am a novice with the whole family history thing, but recently had an Ancestry DNA test done. Family history had my great great grandfather as being Carl Johnson, who was said to be a Norwegian sea captain. Through the DNA test, I have a strong hit of Norwegian blood (suggesting ancestors came from the Hordaland region of Norway). A 'found' relative has information dating Carl's birth to 1831 and his possible death in 1891 (no death certificate located). We do know he had two children in Australia with an Irish woman, Martha Bouvaird (Bouffard) - my great grandfather Nicholas (born in Sydney 1857) and daughter Martha Mary Ann (born in Sydney in 1864). Carl's wife/defacto (?) Martha Bouvaird died in the same year their daughter was born, so it is likely Carl had other partners. Any help, or suggestions about where to look, would be gratefully received. Thanks, Chere.