I wonder if the connection goes further back in SA?
There are several births in Adelaide SA in 1840s to John JOHNSTON and Jane SAVAGE. Alexander 1840, John 1843, James 1846, John 1851, Mary Ann 1855.
Jane Savage JOHNSTON died in 1874, nok John JOHNSTON (husband), reg 59/158, Nairne district SA.
Do you know when Alexander JOHNSTON died and if he married?