OK....going sideways again......and posting the BISA information for Robert Underdown's brother Thomas......
UNDERDOWN Thomas
Parents = Thos and Ann nee HARDING
Baptised 1st July 1821 Southleigh Devon England
died 26th November 1876 Portland Estate SA
buried Alberton SA
arrived in 1849 on the DERWENT*
occupation = Mariner, Labourer, Miner,
Carter, Farmer
Resided at Pt Adelaide, Hindmarsh, Portland Estate
Religion = C of E
Married 30th Aug 1851 Adelaide SA Rachel nee HARPUR
born c1831 WIC Ireland
died 17th Dec 1904 Portland Estate SA
arrived in 1850 on the SULTAN
Children =
Jane (1854-1854),
Thomas(1855-1923),
Rebecca Jane JOHN (1857-1927),
William (1858-1860),
William John (1860-1861),
Anne Elizabeth MATTHEWS (1862-1938),
William George (1863-1916),
Robert John (1867-1917),
Arabella (1869-1870),
Mary Eleanor IRVINE(1 872-1936)
Don't know if this will help us but you never know..... 
......dee
The info in the email is a match for this dee.
This is the notes for Robert
Robert had seaman’s ticket 18287 issued in London on the 6/3/1845. He went to sea as a cook and deserted in St John on the 30/6/1845. He then went to Australia.
A Robert ‘Underground’ was admitted to Kyneton Hospital on 13.10.1882. He was 57 years old, a labourer born in England, married and his residence was at Tylden.
This is the notes for Thomas
Thomas joined the Royal Navy in 1838 for 7 years as a seaman. Seaman’s ticket no 389198 issued in London on the 9/12/1847. He went to Australia. His name is recorded incorrectly in the arrivals from the ‘DERWENT” as Edward Underdown.
Rachel was hospitalized for Tuberculosis on 20/10/1869 but survived and was discharged on 8/11/1869. She was hospitalized 3 further times (in 1874, 1887 & 1895) for ulcered legs.
Thomas died of softening of the brain. I wonder what that would be called now?
Deirdre