Hi Peoplesman. I thought I'd throw my 2 cents worth into the mix.
The rules are: Unless you are working on a claim, you are only allowed to fossick in a public fossicking area, all of which are in the South Island.
If you are not going to comply with the rule, then you need to decide whether the land you are intending to go to is public or private. A good place to start is
http://wams.org.nz/wams/ which outlines public lands and public access. (Thanks to the late Criticol for this site)
There is relatively easy access to most of the larger waterways on the Coromandel. Unlike the South Island, there is no flood gold in the loose gravel. You need to get down to the bedrock. You will need a crevice scraper and a sucker in your arsenal of tools. If you go to the section of this forum - Equipment - Light, "Yet another Crevice Sucker design" posted by Kiwisouth, you will get the idea for a very cheap and very effective sucker.
If you use only handheld tools, I very much doubt anybody will bother you except to be interested in what you are doing. At worst, you may be asked to desist, but I doubt it. You will not lose your car or equipment. If you use a dredge, that may be quite a different story. If you are like most fossickers, you are looking for a reason to go somewhere and something to do after you arrive. Any gold you find is a well deserved bonus.
Get out there and do it. If you get some shotgun pellets or a bit of mercury, you are doing things right. A few colours may follow.
Lammerlaw had good advice. Go south for a holiday. The gold is certainly more readily obtained, but of course there are more sandflies there also.
Good luck with your quest.