Complex sentence: Blacktown City and Sydney United won the NPL national championships in 2015 and 2016 respectively, therefore were not required to participate in FFA Cup preliminary rounds in 2016 and 2017.
Split into simpler sentences: Blacktown City and Sydney United won the NPL national championships in 2015 and 2016 respectively. Therefore the two teams were not required to participate in FFA Cup preliminary rounds in 2016 and 2017.