The Justice and Electoral Committee has called for submissions on the New Zealand Flag Referendums Bill. Submissions can be made online at the link above, and are due by Thursday, 23 April 2015. If you're not sure how to make a submission, there's guidance in English and Maori here.
If you're wondering what to mention, I'd start with the lack of spending limits on advertisers. These are present in every other piece of electoral law, and clearly contemplated, but appear to have been removed. They need to be in force to ensure a level playing field and prevent anyone from attempting to buy the result.