Complex sentence: Berejiklian's only serious leadership challengers--decided not to contest the leadership, and instead opted to support Berejiklian, allowing her to be unopposed at the ensuing leadership contest held on January 23.
Split into simpler sentences: Berejiklian's only serious leadership challengers--decided not to contest the leadership, and instead opted to endorse Berejiklian. This left Berejiklian to take the leadership unopposed at the ensuing leadership contest held on January 23.