The Last Circulating British Halfpenny Of 1967

My speculation not based on evidence: (don't know for sure):

The Bank of New South Wales (now Westpac) was ordering shipments of new British coins into Australia from 1825.
In the period 1825-1911, British Farthings did not circulate in Australia, but Halfpennies did.
David Graham with his detectorists activities supports this with the complete lack of discovering any British Farthings.

A decision must have taken by the Board of the Bank of New South Wales in 1825 and continued until 1911 not to import Farthings. Minutes of Board meetings of decisions taken waaaay back in 1825 no longer exist.

The Commonwealth Bank of Australia (which took over the responsibility for issue of coins and Notes in 1910), must have followed up on this decision not to circulate Australian Farthings, most probably at the direction of the Australian Government Treasury.