Here's my 2 cents on these issue guys.
First off NZ needs SOME ability to control it's own airspace. The incident a while back with the madman who wanted to fly a light plane into the tower in Auckland demonstrated this. NZ also needs a light aircraft to conduct basic flight training for P-3K, C-130H and B-757 crews.
Perhaps something like the Embraer Super Tucano or Pilatus PC-12 could be employed? Such an aircraft would provide excellent flight training and can be armed with basic weapons packages including machine gun/light cannon pods, rocket packs and light free fall bombs. Such an aircraft would give a basic capability to conduct air intercepts of a low level nature. (they are used in South America primarily for this type of work) and the PC-12 has the same performance as a Hawk Mk 127 LIF...
In benign strategic situations, they could also be used for COIN or CAS for low level ops in support of light inf peace-keeping forces etc.
These are VERY cheap aircraft and would hardly "break the bank" yet provide a modest, but useful capability. It would also mean RNZAF could provide reasonable air defence training from it's own resources for NZDF air defence elements. Something it cannot do now.
As to the anti-ship missile question. NZ HAS a plan to acquire such a capability under it's LTDP, with funding set aside. Presumably this is primarily designed for the planned ANZAC frigate upgrade, however there is no reason WHY such a missile could not also be employed by the RNZAF P-3K fleet.
Arguing that you "don't have the equipment to support such a weapon" is a non-sequitor. Of course you don't. You don't have the weapon yet, why would you have the equipment to operate it? The acquisition of such a weapon WOULD include all necessary support, development and training items needed to effectively operate the capability.
Some information on the P-3K platform might also provide some insight on this topic too.
The P-3 Orion has a maximum take-off weight of 61,235kg's. It can carry 34,800 litres of fuel internally and has a maximum range of 3800k's. It can however manage to remain on station for a total of 3 hours (loitering capability if you like) to a range of 2500k's. Since 1976
ALL P-3's built have been capable of employing the Harpoon weapon system. RAAF AP-3C's have done so routinely since the mid-80's when Harpoon was first acquired by the ADF. The Project Rigel upgrade given to RNZAF P-3K's in the mid 80's provided them with the basics necessary to operate such weapons. Some integration would probably be required, but would hardly be difficult given the preponderance of P-3/Harpoon combo users in the world.
Although the RNZAF's further L-3 upgrade package has concentrated mainly on it's maritime surveillance capability, it's ASW and ASuW capabilities remain. They simply are not being enhanced. There seems to be no technical reason therefore WHY RNZAF P-3K's could not easily employ the Harpoon or Harpoon Block II weapon systems. All P-3 Orions are capable of carrying a maximum of 4x Harpoon ASM's on under wing and under fuselage pylons. RNZAF's are no different. Here's a photo to illustrate this.
http://www.airforce-technology.com/projects/orion/images/orion11.jpg
Given a lack of technical reasons why such a weapon could not be employed by P-3K. We are left with strategic rationale and financial reasons for such a weapon to be procured.
In my opinion, the addition of a standoff weapon system for P-3K is the best option for RNZAF to regain a maritime strike capability. The acquisition of a tactical fighter to conduct this and other roles, is a virtual impossibility given recent NZ Government and opposition party statements. However an ability to control the SLOC around NZ remains a fundamental responsibility if the NZDF. A competent maritime strike capability would go a long way to ensuring NZ's ability to control such an area.
The question then exists, how else could NZ acquire such a capability? 3x ways are feasible at this time. For the ANZAC class frigates to be provided with such a capability (as is already planned) and for NZ's Super Seasprites to be provided with such a capability and for NZ's only other combat capable aircraft, the P-3K to be provided with such.
For logistical commonality and budgetry purposes, equipping ANZAC and P-3K with the same weapon, would seem to be the "logical" way to go. however this may incur capability limits. P-3K's with their speed (compared to an ANZAC frigate) offer a far more responsive and flexible capability. It may take days for an ANZAC frigate to be capable of responding to a threat to NZ interests, and this is the case whether they operate 2 or 3 frigates.
Seasprites already have a limited capability with the Maverick AGM. this system however has limitations as the Seasprite requires either an ANZAC frigate or MRV to deploy aboard. It also has the significant range and speed limitations common to all helo's. The Maverick also has significant range limitations and it's warhead size and design are also factors in the ASuW role. A cheap option to provide a limited ASuW capability to the P-3K might be to integrate Maverick onto it. However range limitations will still apply as will it's suitability in the ASuW role. Land strike missions would also be possible with a Maverick armed P-3K however as you rightly point out, platform survivability then becomes an issue.
A P-3K armed with Harpoon however, could reach up to 2500k's away from NZ mainland and conduct a strike within hours. If Harpoon block II were acquired, P-3K's could conduct littoral and "limited" land strike missions ie: (coastal radar/communication/air defence sites etc). The Advanced Harpoon Weapons control system (AHWCS) needed to fully employ the Block II is relatively inexpensive and easily installed, from all reports, but offers considerable flexibility as far as targetting options go. The P-3K's existing sensor fit in any case are capable of providing the necessary targetting data for Harpoon missile shots anyway.
As for the P-3 vulnerability, USN and RAAF P-3's have operated in "overland" surveillance missions in Iraq for years. How many have been shot down? Iraq is a vastly more uncertain air environment than the South Pacific, yet the users have flet comfortable employing them in that situation. RNZAF P-3K's have deployed to the Gulf numerous times as well and been perfectly safe their as well.
As to the strategic situation in the South Pacific. No Country there (besides French New Caledonian forces) has any form of airforce whatsoever. Most don't even have any helicopters of any kind. None have any naval forces capable of operating an air defence system and none have any kind of SAM system, let alone advanced Russian or Chinese made SAM systems, capable of threatening an aircraft with 100k+ standoff weapons.
IF the situation worsened and SAM systems were introduced somewhere, do you think NZ would not respond? EWSP systems are extremely advanced at present and designed to protect aircraft from precisely the kind of threat you envisage. It doesn't make them impervious (as shown by the shooting of down of the RAF Hercules in Iraq) but it makes them very hard to kill.
Planning on your force structure and denying yourself an effective capability based on a potential as opposed to an actual threat, seems ludicrous in the extreme. Isn't the idea of military capability to eliminate potential threats, rather than kowtow to them?
Without any kind of air delivered strike weapon to counter a SAM threat, your B-757, C-130H and NH-90 fleets are going to be pretty exposed when they attempt to land light infantry/special forces elements. Relying on Australia is a dangerous proposition too. We barely (and possibly don't) have enough capability for our own needs. It is extremely unlikely that we will have sufficient capability to cover others, who refuse (despite being capable enough) to provide for their own...