Red-crowned (type) Parakeets

Hi all, I was going through the new IOC/BARC list and noticed that the (extinct) Lord Howe Parakeet and Macquarie Parakeet have been lumped into Red-crowned Parakeet, yet the extant Norfolk Parakeet remains a separate species. I haven’t checked the world list for the status of the other parakeets on the NZ islands, but was wondering if anyone could link me an article or otherwise explain how it is that Norfolk is a whole species, yet the Lord Howe bird some 1000km from Norfolk is different, but the same as the Macquarie bird, some 4000km away. Thanks, Ashwin