This is not a government department running a social welfare service
Yes on some particular flights Baa Baa which are close to fully booked AIR will charge you a top price if you are insistent on travelling on that particular date and on that particular flight. Pragmatic people simply accept the commercial reality that ALL airlines are there to maximise profits, (welcome to the commercial reality of the aviation world), and take steps to mitigate the effects thereof. In your case this could involve changing flight times or date or simply driving the short distance to Wellington airport and flying out of there. 50 km's and 40 minutes is all what I hear is involved, more during the rush hour...roughly the same amount of time it takes me to travel to Auckland airport every single time I fly.
Frankly I'm a little surprised that AIR N.Z. bother to provide the good citizens of Paraparumu with the super convenience of flights right from their back doorstep with such a major airport in such close proximity. Be careful what you wish for because if AIR can't make a profit on the Paraparumu flights you'll lose that convenience.
If you are price conscious there are ample cheap flights from Wellington to Auckland and vice versa for those that book ahead a bit so for the sake of a relatively short drive I really think you already have the power to do something about it if the flight prices out of the Kapiti Coast concern you so much. On the other hand if you're an extremely busy businessman and value the extra time each way saved on a commute to Wellington airport then pay-up and be glad you've got that super doorstep convenience that most of us don't have the privilege of being able to chose to enjoy.