William Baxter - Throndhjem

I am looking for information on a William Baxter whose Marriage certificate of 1 Feb 1883 in Qld Australia has his birth as Throndhjem, Norway. We have estimated his birth to be between 1843 and 1845. His marriage certificate has his father as Andrew Baxter and mother as Edith Forster. We have never been able to work out whether William was really Norwegian or not as I have found many Baxters in England and Scotland (as well as Forsters) and wonder whether he may have been born during his parent's emigration to Aust. To date I have been unable to find record of William coming to Australia and/or Andrew and Edith. Any help would be greatly appreciated. Kathy