In case you didn't watch Fair Go:
Car Registration is going down in price from July 1 2015 - by as much as 41% for most petrol cars (due to reduction in ACC Contributions)
Fair Go suggested (if you need to renew before July 1) only paying for up to July 1, apparently you can get car registration a a single day
Looking at online and paper renewals the minimum amount of time is 3 months - couldn't find a 1 day option myself
Anyway good news about the price coming down