The Ships Passenger List on the NSW.gov site.
Aurora. Arrived Port Phillip 7 Dec 1848.
William Douglas, 32, Shepherd, b. Lowick Northamptonshire, Presbyterian, can read and write.
Isabella Douglas, 25, wife, b. Lowick Northamptonshire, Presbyterian, can read and write.
Thomas Douglas, 1, son, b. Lowick Northamptonshire, Presbyterian.
Infant son Douglas, born on the voyage.
There is a Lowick in Northamptonshire, but not likely to be many Presbyterians there, so probably a mistake.
Info supplied by Christine in a post last year, in a thread started later than this one.
"William son of Thomas Douglas and Isabel his wife of this parish of Lowick, county of Northumberland, born July 13th 1816 was baptised July 15th 1816
Entry in Presbyterian records for Lowick"
Right name, right age, right place, right religion, so thats almost certainly him. Might be a record in there for son Thomas baptism also, unless he was baptized in Scotland.
Alan.