A think the regulations state only the winners of either the Canterbury Premier League and the Nelson Bays Premier League are able to seek promotion subject to meeting the other requirements.
As per the regulations:
Playoff Series: the competition managed by Mainland Football to determine promotion into the Southern League including the winning teams of the CPL, Nelson Division One and Southern Premier League.
The highest ranked team on the CPL final table will be nominated for the ‘Playoff Series’, to compete for promotion into the SL.