Bear in mind that Hobbiton is a good 2hrs drive from Auckland, but if you already know this then you'll be fine. But this is what makes the trip somewhat pricey when going direct from Auckland just for the entire day.

This trip starts from $295 per person, for the day: https://www.newzealand.com.au/tours/hobbiton-express-tour

Better yet, Naked Bus (legit, don't worry!) has day trips from only $139 per person: http://nakedbus.com/nz/home/day-tours/hobbiton-day-tours-2...

Or Intercity business has one too for $159: https://www.intercity.co.nz/tours/intercity-hobbiton-excursion-auckland-to-auckland

For yet more options, check out the official Hobbiton webpage: http://www.hobbitontours.com/en/getting-here/transfers

You could always catch the public bus to Matamata and stay overnight if you prefer. Lake Karapiro Lodge is nice and not too far (20kms via taxi) from the Hobbiton film set. Just a thought... In any case, have fun and why not post up a little trip report around here!