Thanks for sharing your old cool document.Neither of the 2 names of witnesses appear to be Norwegian (although they could be Anglicized or other -ized forms, not enough information to there to know).
My experience in America is that name changing is a extra legal process, that is very seldom involving a legal procedure but more the result of an adhoc process.The same person could have a variety of variations of their name through time, Uncle Sam might know them by a name, their pastor by another, the neighborhood yet another.Some folks wanted to "fit" in and so followed a more Yankee sounding name, others simplified their Norwegian name so Yankees could pronounce and write it, some translated their Norwegian name into an English name, others picked a Yankee name which sounded like their Norwegian name.O so many variations possible.
I see that they were married by a Catholic priest. So there is at least a possibility that there is a church record of their marriage in addition to the civil record. There are exceedingly few Norwegian Catholics (the state religion in Norway is Lutheran) so the parish priest of Goolwa may have been inclined to record a bit more about the "outsider and unbeliever" than he might have otherwise. Perhaps their children were baptized R.C. also?It's possible that more information might be included on the parents in the children's baptismal records.There could even be more information on the children's death or funeral records.(I know of a case here in US of an old Polish guy, father of 12 children.Descendants all knew he was from Poland but actual point of origin is only listed in one place, not in the life records of the old man himself but in the church register of the funeral of one of the last children--a pretty thin margin considering the vast progeny!)
I'm not experienced in Australian research but did have one Norwegian family which moved to Australia in the late 1890s and one of their children changed their last name (which came from the name of the farm of origin of the dad of the family--which was pretty unique and ethnic sounding) to a completely English sounding name.Found that reference at The Trove site.
As I said, scrupulous document collection may be the best hope for resolution.