Complex sentence: A redistributions of boundaries of and divisions in regions took place in 2015, which was in effect for the 2017 State election held in March 2017, but which takes effect in May 2017.
Split into simpler sentences: A redistributions of boundaries of and divisions in regions took place in 2015. These changes were in effect for the 2017 State election held in March 2017, but will take effect in May 2017.