A by-election was held for the New South Wales Legislative Assembly electorate of West Sydney on 17 January 1866 because John Robertson had been re-appointed Secretary for Lands in the fourth Cowper ministry. Such ministerial by-elections were usually uncontested however on this occasion a poll was required at both West Sydney and The Williams (Marshall Burdekin). Both Robertson and Burdekin were defeated, with the Cowper government falling, replaced by the second Martin ministry.
<includeonly> John Robertson was re-appointed Secretary for Lands in the fourth Cowper ministry.</includeonly>