According to some online trees Stephen Thomas Shears was born circa 1830 Plymouth, Devon and married Mary Halden in 1858 in Victoria, Australia. The marriage registration gives him as Stephen Thomas Shears, not plain Stephen.
There is only one that seems to fit in any way - 1841 Census for East Stonehouse, Devon shows:
Stephen Shears 35 Lab. Not born in Devon
Susan Shears 35 " " " "
Mary Shears 14
Harriott Shears 11
William Shears 9
Stephen Shears 7
The children all born in Devon. The parents married as Stephen Shear and Susanna Harris 16/5/1826 East Stonehouse, Devon. From the trees and other items I've looked at the children seem to be (as above):
Mary Ann bc.1827 (married a William Saunders)
Harriet Elizabeth bc.1830 (said to be married to a Henry Murch, an Evangelist, but can't locate)
William Henry bc.1832 (married in London 1871 - sister Harriet Murch a witness)
Stephen bc.1834 who appears to be Stephen Thomas.
Stephen Shears senior appears to die in March quarter 1851 East Stonehouse - in 1851 his widow Susan has married daughter Mary and her family with her.
Looks like Stephen junior received 3 months for larceny in 1851 and then he disappears in UK records so fits with being your Stephen Thomas Shears who married 1858 in Victoria.
Unfortunately, East Stonehouse (Plymouth) baptism records don't appear to be online for the relevant period.
Stephen senior is said on 1841 census to have NOT been born in Devon, likewise his wife. I cannot find a likely baptism for him anywhere in England. His wife, Susan/Susanna, states on 1851 census that she was born South Hill, Cornwall.
Annette