Q: Is there a government subsidy I can apply for?

Unfortunately no, there was a government grant scheme run through the EECA Energywise program, however that scheme has now closed, as of the 14th of June 2012. However there are other subsidies available if you have a Kiwibank or NZ Homes Loans mortgage.
See https://www.kiwibank.co.nz/personal-banking/home-loans/rates-and-options/sustainable/ or http://www.nzhomeloans.co.nz/Home+Loan/Sustainable+Energy+Loan.html