Given that they married in Oatlands, there is a possible lead in the newspaper at NLA Trove, being the marriage notice of an Alfred NICHOLS, youngest son of Thomas NICHOLS of Oatlands, on 10 Feb 1875 at Hobart Town. Seems to fit the timeframe.
There is a post about this family online:
>I was wondering if anyone had information on Thomas Lewis Nichols born England 1801 and died Oatlands Tasmania in 1882.
>He was married to Mary Ann Vincent, daughter of John Vincent builder of the Oatlands flour Mill and a number of hotels in the district.
NICHOLS.—On the 6th instant, at Musgrave Cottage, Oatlands, Tasmania, Thomas Nichols, aged 81 years.
This family is listed in the Tasmanian Colonial index but sadly Agnes is not listed among the children. Still, could be a lead there somewhere as there are others researching this family who may have info. Agnes is there - shown as born 1847 - because of her marriage to Samuel.
There was also a George NICHOLS (b. 1822 Hobart) in the area at the time; he was superintendant of Oatlands police c1875. He married Harriet WEEDING at Oatlands 1847. Agnes is not listed among their children but could have been born prior to their marriage?