Indust,
I believe that you are well aware of the downsides of contacting potential manufacturers in an early stage (i.e. before you have filed for a patent application). The interest you can get by showing not the invention itself but derivatives (products?) from it would depend on the type of invention, the scientific field in which it resides, the economic value of the invention and the willingness of industrial partners in the filed to innovate. These factors are unknown to me, and could be best judged by yourself as expert in the field (but try to be objective).
Further - as non US resident - I should provide an additional comment to the proposals for first filing a US provisional application. For a New-Zealand inhabitant, I do not see any advantage of filing such an application. The main reason why I am advocating filing a PCT application in your case is:
a) you will still get an early US filing date (the PCT application counts as a US application); and most improtantly
b) you will get a novelty search and a first opinion on the patentability of your invention during the first year after filing.
This latter will not be provided by a US provisional filing and I suppose in your case it is of the utmost importance to be sure* to have a patentable invention to continue the patenting process.
Also, as you may already have learnt from the responsesn in this thread, it is very important that your patent application is well drafted. This is something for which you will need help from an experienced patent attorney or patent agent. It is better now to spend some extra money to get things done in a correct way, then later be sorry. You got only one chance to do it right.
Success
*Be aware that you can not be 100% sure, since the search can - by definition - not be 100% complete.