Based on the scant details you have given I think the Mary MUMFORD in question was married to Samuel James NORTHEY. If so her full name was Mary Jane MUMFORD.
They arrived in Victoria in 1869. They had a daughter born at sea on 9 Dec that year (unnamed on marine reg, 2), aboard 'Charlotte Gladstone'. Certificate shows Samuel (age 26) was born at Calstock, Devon and that they were married 18 Sep 1866 in Tavistock Devon and had 2 children, both living. Mary was aged 24, nee MUMFORD, and her birthplace is shown as Ivy Bridge, Plymouth, England. Travelling with them was 2 year old John H and Mary A NORTHEY aged 62, presumably Samuel's mother.
Samuel & Mary later moved to Gordon, Victoria.
Children born in Victoria:
Mary Louisa 1872/6922, d. 1935/12743 Hville aged 62 [as HODGES]
Charlotte Gladstone 1874/8949, d. 1956/22567 Ballarat aged 82 [as BARNETT]
Elizabeth 1876/16608
Samuel James 1879/3118, d. 1942/10034 West Brunswick aged 64
William 1881/9922, d. 1884/12470 Gordon aged 3
Josiah 1883/23814, d. 1953/4638 South Melb aged 68
Annie Beatrice 1886/10703, d. 1972/24817 Melb aged 86 [as STONE]
You will probably never find anyone who 'knows why they moved there', apart from the lure of gold at the time since the goldfields seems to be where they headed first. It might help to find out what Samuel's occupation was.
Samuel died in 1892 in Gordon, aged 48, son of Wm & Mary (reg 2159).
Mary died in 1920 in Ballarat aged 75, daughter of John MUMFORD, mother unknown (reg 12911, as Mary Louisa). Death notice has correct name:
The Argus, Saturday 9 October 1920
NORTHEY.-On the 7th October, at Ballarat, Mary Jane, relict of the late Samuel Northey, of Gordon, loved mother of John, Mary, Charlotte, Sam, Joe, and Annie, aged 76 years.
"Thy will be done."
http://nla.gov.au/nla.news-article4578866Eldest son John Henry died in 1940 aged 72 in Kerang (reg 15932).
See also
http://nla.gov.au/nla.news-article60452025http://nla.gov.au/nla.news-article11995152http://nla.gov.au/nla.news-article11996193http://nla.gov.au/nla.news-article65382008Interested to know if this seems like the right Mary to you since your other thread shows her as Mary Ann. ???
PS There are articles at NLA Trove that show Samuel was a gold miner - so that indicates that perhaps they did move to Victoria for that reason.