Hi
Robert Thomas Turnbulls birth is registered in Gateshead June 1855 mother' maiden name Oliver Vol 10A page 491
You could order the birth certificate which would give his parents but let's try without.
Other children Turnbull/oliver- Margaret Newcastle u Tyne June qtr 1852.
Baptism Robert Turnbull at Gateshead 6 Oct 1856 age 1 to John Turnbull and Isabella Ann
John Turnbull married an Isabella Ann Oliver at Newcastle Mar qtr 1855 vol 10B 98.
If you buy the marriage cert it would give you their fathers.
It was Newcastle St John the Baptist if someone can look locally
Isabella Oliver it seems appears on the 1851 census for Newcastle New Road(altough her age is lower)
Isabella Oliver 42 butcher's widow born Kendal
Ann (Isabell Ann) 21 born Newcastle
Isaac 6
She was baptised Newcastle 23 Aug 1835 d of Thomas and Isabella Oliver.
Other children Mary Ann 19.11.1827,Thomas 20.6.1830,Thomas 15.8.1831 Isaac 14 Sep 1845,Jane 16 Aug 1846.
John,Isabella and their children don't seem to appear on the 1861 census so that arrival in Australia in 1859 seems right.
It gives father John 24 labourer and his wife Ann 23,Robt Thos 3
That would fit with what we have.
So John Turnbull would be born c 1835
I'd love to give the info for John but there are several choices.
One family tree has a baptism at Tynemouth 20 Sep 1835 to William and Isabella
The 1851 census has a Robert Turnbull son of either William and Elizabeth living at Gateshead or John s of William and Margaret also at Gateshead a stone mason or John F Turnbull s of William and Ann at Newcastle a labourer.
If you can find his death in Australia hopefully it should give his parents
Alternatively you may have to buy that marriage in 1855 ,now c £7 for an electronic copy.
If i copied anyone's else's sorry but i was typing
Ciderdrinker