This is what I found on the records at the NLA
Mary A THURSTON, 18 arr MAR 1857 per JAMES FERNIE, from a British port.
Fiche 121, p008 shows:
THURSTON, Mary A, age (listed in column for single women) is difficult to read - could be 14, 15 or 18 with the units figure being faintly overwritten so that it looks to me definitely that the record shows in her teens, and 18 seems more likely. Occupation is listed as "Cook" and she is from England. Unfortunately this record only shows whether the person is English, Scotch or Irish, so no locality information.
This one from the Disposal List (Book 11, p338)
THURSTON, Mary Ann, Cook,
Native place: Suffolk
Curch of England, can read and write, age 28 (clearly written)
Disposed to: Mr (perhaps Mrs?) Stephens, Prahran on the 17th; wage: 35 (presumably pounds). The wage is higher than that for the women listed as General Servants.
Now for the women on the
Theresa.
From Bk 13A, p 251, per ship
TheresaMCCartney, Hester B, 18, housemaid,
Employer: Mrs J Franklin, 43 Collins St West, Wages 12 (pound)
From Bk 13A, p 252, per ship
Theresa (Cando has already listed this)
THURSTON, Mary Ann, 22, housemaid
Employer: Mrs Perks, Parsonage Richmond, Wages 20 pound
From Book 14 p265
THURSTON, Mary Ann, housemaid, 24
Native County; London, Church of England, can read and write
Employer: Mrs Perks, Parsonage Richmond, Wages 20 pounds
MCARTHY, Esther B, housemaid, (I am sure this is the same person as Hester MCCARTNEY))
Native County; London, Church of England, can read and write,
Employer: Mrs Franklin, 43 Collins St West, Wages 12 (pound)
The woman listed next, under Esther MCCARTHY is Euphemia McDONALD from Perth Scotland and is also employed by Mrs Franklin.
So we are no further forward, except that we now have two possible Mary A THURSTONs

However, the one who arrived in 1857 has a 'native place' of Suffolk which does not match any of the information for the second Mary Ann. There is no record to show whether H/ester B MCARTNEY/MCCARTHY knew Mary Ann THURSTON but surely the co-incidence of them both marrying in Beechworth is of some importance.
Judith