Hi there
Mary Ann Rickard married Alfred John Knight as per your info.  She died in 1891, only 39 years, of typhoid fever in Hobart hospital.  Alfred died just a couple of months early, cause unknown - age 41.  They left about 10 children, maybe more.  However several of them died young.  One of their children was Lillian Knight (1886 - 1966), my great grandmother.  Lillian married Orlando Patterson (1891 - 1969), and had 4 children.  One of these was Betty Patterson (1925-1993) - she was my grandmother.  She married Colin Hankin (1924-1960) and had 1 child, my father Phillip.  Roy Patterson, the son of Alfred Patterson (one of Bettys brothers) is still alive and has done some family research. 
With a few adult children the descendents of Mary Ann Rickard may be numerous.  My information comes from the family and ancestry.com.au, but I haven't uncovered all the relatives.
Any information you have on Mary Ann Rickard's mother, would be great.  I cant find much information about her,  the only verifiable information is a marriage record for  Mary Reid who married William Henry Batavius Rickard on 14 Aug 1849.  I have come to doubt her middle name as Ann, as Williams second wife was Mary Ann Kitchingham, maybe too much of a coincidence?!
Cheers, Chris