Is the use of helicopters a permitted activity? And why is this significant?

A permitted activity is one that is permitted by a District Plan or that has already been consented to.
The argument has been made in Resource Consent hearings that since it is permitted for helicopters to hover over a vineyard to control frost (by performing the same physical action that a frost fan does), then a frost fan can be permitted since the noise from a helicopter is greater than a frost fan.
This argument has been tested in the Environment Court and is not valid.
In Maclean vs Marlborough District Council (C 081/2008, para 25) the Court found that;

"The use of helicopters as wind machines is not said to be permitted by the resource management plan or by any resource consent. It therefore cannot be part of the permitted baseline."

The Resource Management act cannot be used in respect of noise emitted by any aircraft being operated during, or immediately before or after, flight (RMA 326(1)(a)).
Likewise the Civil Aviation Act only regulates in matters of aircraft safety, not noise levels.

As it was put by the Court of Appeal in Queenstown Lakes District Council vs Hawthorn Estate Limited (2006, NZRMA 424 at para 65);

"It is well to remember what the "permitted baseline" concept is designed to achieve. In essence, its purpose is to isolate, and make irrelevant, effects of activities on the environment that are permitted by a district plan, or have already been consented to. Such effects cannot then be taken into account when assessing the effects of a particular resource consent application."

So, the upshot of it all is that if the argument is made that since helicopters can be used to fight frost, frost fans should be able to, there is a precedent that says that the argument cannot be made on the grounds that helicopters form a permitted baseline.
It may be valid using the argument of the 'receiving environment', but that post will have to wait for another day.