Okay, i've done some reading about this. By the sound of it as long as the gear is visibly clean AND dry AND declared when you arrive, you shouldn't have any issues.
I've been reading on this link:
http://www.biosecurity.govt.nz/imports/animals/standards/anieqpic.all.htm
This is the relevant part of it:
6.3 Used equipment associated with marine aquatic animals or marine water activities from any country may be given biosecurity clearance provided all of the following requirements are met:
i The equipment is visibly clean and visibly free from contamination with organic material such as algae, animal parts, blood, faeces, fatty tissues, flesh, fungal material, hair, plant parts, seeds, sediment, soil, weeds and biofouling or other marine organisms
AND
ii All parts of the equipment are completely dry on arrival to New Zealand
Note:
Used equipment associated with marine aquatic animals or marine water activities include aquaculture equipment, aquaria equipment, diving equipment, fishing equipment, surfing equipment, canoes, kayaks, boats, nets, traps, ropes, buoys, and pontoons.
This list is not exhaustive.
6.4 Used equipment associated with freshwater aquatic animals or freshwater activities from any country may be given biosecurity clearance provided all of the following requirements are met:
i. The equipment is visibly clean and visibly free from contamination with organic material such as algae, animal parts, blood, faeces, fatty tissues, feathers, flesh, fungal material, hair, plant parts, seeds, soil, weeds, wool and biofouling or other aquatic organisms
AND
ii. All parts of the equipment are completely dry on arrival to New Zealand
Note: Used equipment associated with freshwater aquatic animals or freshwater activities, containing absorbent material (such as nets, traps, ropes, leather, fabrics, cloth, neoprene, and other material that traps water when wetted) and non-absorbent material includes aquaria equipment, fishing and angling equipment and clothing, boating equipment, canoes, kayaks, boats, buoys, and pontoons.
This list is not exhaustive.
And then it says.....
PART C. CLEARANCE PROCEDURE
7 BIOSECURITY CLEARANCE AND DIRECTION
7.1 Upon arrival in New Zealand, the consignment must be inspected by an Inspector at the port of arrival.
7.2 Where an Inspector is not satisfied that the consignment meets all of the requirements stated in the Eligibility section of this import health standard, they must:
EITHER 7.2.1 Treat the consignment at the port of arrival with one or more of the specified treatments as outlined in Tables 1, 2, 3 and 4 of this import health standard
OR 7.2.2 Issue a biosecurity direction under section 25 of the Biosecurity Act 1993 authorising the consignment to be directed to an approved treatment facility where it is subject to one or more of the specified treatments as outlined in Tables 1, 2, 3 and 4 of this import health standard
Thats most of what seems to be relevant. Those are the regulations and i don't see anything in there about destroying items - from what i have read so far that is.
Hope this helps a little.