Hi Raina,
Possible Marriage from FreeBMD;
John Spearman SHEPHERD
December 1838
Plymouth
9 492
Mary Ann STEPHENS
Now I have found this combo with a son William aged 9 , arriving in NSW 1849 but if the 1851 Census posted is correct it cant be them.
Ship, St Vincent
Arrived Sydney and/or Newcastle, 1849
John S SHEPHERD, 32
Mary Anne, 40
William I, 9
Mary Jane, 3
James, infant
From State Records NSW, Shipping Arrivals
Do you have the Marriage Cert of William and Agnes??
If not it is important to purchase to confirm his details and Fathers name and occp otherwise we could be chasing the wrong person.
Marriage, Shepherds Plain, NSW;
William J SHEPHERD
Agnes HOWLEY
2866/1864
So it is Howley for Agnes and William is William J. which could be mistranscribed as I on passenger list.
Then to add to the confusion the birth reg on their 8 children born Maitland have him as William T or William Thomas

And one child has mother as Agnes Thorley.
I think the Marriage Cert is a must!!
Plus Williams English Birth Cert.!
Cheers,
Trish