There would be nothing illegal about it at all.
Perhaps, perhaps not. Section 97 of the NZ Employment Relations Act 2000 may have something to say about that!!!
Employment Relations Act 2000 No 24 (as at 01 July 2013), Public Act 97 Performance of duties of striking or locked out employees ? New Zealand Legislation
Think it would depend on the company setup and who is deemed the employer. Are the pilots employed by a NZ company or the Australian company.
In other words is the employer the same for the NZ and Australian pilots?
If they are the same employer (from a company entity viewpoint) then Aus pilots may be able to be brought in. If it is deemed a separate employer in NZ then The Australian Pilots will be illegal labour and will find themselves in breach of NZ employment law.
Of course this would no doubt need to be proven in court and by the time it is resolved the "foreign" labour will have had the desired effect to the detriment of the VANZ pilots.