According to the MoD the RNZN frigate replacement programme is earmarked to formally start in 2019 though an RFI wont likely be released until the middle of next decade.
From my POV, the replacement frigate programme timeline is going to be dependent on just how much work has been done prior to the formal start of the programme.
Using some of the recent historical Oz/Kiwi capital defence projects as a guide, it seems to take on average about 14 years between the formal start of a programme and IOC, assuming the acquisition is a planned acquisition as opposed to an 'emergency' acquisition.
Looking at Oz and the SEA 5000 programme (in terms of the timeline of events only) there had been mentions of the need to replace the RAN's
ANZAC-class FFH's in both the 2009 and 2013 Defence White Papers which were both well before the formal start of the SEA 5000 programme in 2015. This was not unlike how the subject of the Future Surface Combatant to replace the RNZN's
ANZAC-class frigates has been mentioned in the 2016 Defence Capability Plan, which was anticipating that programme starting sometime in late 2018, and with procurement at least well underway if not necessarily actually in service by 2030. I have taken that to mean that it is expected that the future design and construction yards will have already been settled on by 2030, with the potential for first steel to have already been cut as well.
Again, looking to the SEA 5000 timeline, the programme formally commenced in 2015 and a year latter three designers were short-listed. A year after that an RFT was released, and a little more than a year after the RFT was released, the winning entry was selected and announced.
What this suggests to me is that if the RNZN, NZDF and gov't starts out with a fair (and reasonable) idea of what capabilities are desired in the RNZN's replacement frigates, then selection of the replacement design could occur within as little as about three years, which would be around 2022. If (big IF) that decision is made in 2022, alongside the selection of the build yard, then first steel might be able to be cut in 2025 or 2026 which could see the first of class in service around 2030.
IMO there would be several caveats and concerns about this though. One of the first being the actual degree to which the RNZN, NZDF and gov't start the replacement programme having already established a minimum capability baseline. The more time and resources which need to be committed to the study and analysis of then current and projected future threats and developments, the further back any RFI or RFT will occur, and with it the further back decisions will be made.
An area of significant concern for me, particularly given the history over the last generation of Kiwi defence acquisitions, is that the future frigate is also going start out with a low capability requirement as a baseline. In that respect, it would not be unlike the capabilities the
ANZAC-class frigates started with, which was a limited ASuW capability via the 5"/127 mm gun, limited ASW capability via a single naval helicopter sans dipping sonar, backed up by LWT launchers and a hull-mounted sonar aboard ship, and a limited AAW self-defence capability via RIM-7 Sea Sparrow missiles and a Mk 15 Phalanx CIWS. Here we are, nearly 30 years after the design and capabilities were selected, and such capabilities would now be considered obsolete and inadequate even for just a self-defence capability, never mind if the vessels were required to escort and defend other vessels like commercial shipping. It is unfortunate that I consider there to be real potential that security threats to NZ, especially in the maritime domain, could be ignored either due to ideological limitations decision-makers suffer from, or as a result of ignorance which has been referred to as "sea blindness". If the RNZN were to again get "combat vessels" primarily suited for operating and patrolling in NZ's EEZ, then the RNZN would be degraded to a more Coast Guard or constabulary role. This in turn would be a force ill-suited to Chapt. VII operations should they either become necessary, or get forced upon NZ. Even Chapt. VI operations might be beyond the realistic scope of NZ capabilities, if a low baseline capability is selected.