I'd have to say that I really don't think Manx shearwater is being too optimistic at all. There are already two beach cast records for NZ (according to the 1990 checklist, in my recent move I can't find my new one!) - 1972 Pukerua Bay; 1985 Waikanae Beach. Possibly a third specimen found recently (http://groups.yahoo.com/group/BIRDING-NZ/message/1077). So they have already been found here. A live record would certainly be good, but not impossible, and surely on the cards in the next 10-20 years of pelagic birding. Sav saw what could have possibly been a Manx in the Cook Strait in late 2008 (http://groups.yahoo.com/group/BIRDING-NZ/message/3496), as has Roger McNeill (http://groups.yahoo.com/group/BIRDING-NZ/message/2250).
Having seen them in both the Atlantic and Pacific (more on that in a minute), I'd have to say there would be no problem separating from fluttering, and definitely no problem separating from Hutton's. But at a distance little shearwater may present more of a problem. However, if seen well I don't think they would be that hard at all. Manx have an almost white underwing, with thickish dark leading and trailing edge, which in my mind would be completely different to the slightly dusky underwing of fluttering and very dusky underwing of Hutton's. The very white underwing of little could be more problematic, but a combination of jizz and facial and neck pattern (which of course is very variable depending on subspecies, but never with such extensive dusky sides to the neck), and less of a dark leading edge to the underwing would make separation of the two possible at all but distant views (in which you probably wouldn't be thinking about Manx anyway!).
Manx do exist in the North Pacific on a regular basis. I saw several birds last northern summer off Alaska, and they have been recorded further south along the US coast. I discussed these records with Steve Howell on our recent pelagics and he also had a feeling they may be breeding somewhere up that way, having somehow made their way into the Pacific (probably around Cape Horn, not via the North-West Passage).
The photo of the Manx shearwater is actually one of the Alaskan birds I photographed, for some reason easier to photograph and get close to up there in amongst all our sooty shearwaters! So there is the way in which one will be found in NZ, just search the flocks of returning sooty shears in Sept/Oct.
|Little shearwater, Kermadec Islands|
|Hutton's shearwater, off Kaikoura|
|Fluttering shearwater, in the Hauraki Gulf|
|Manx shearwater, off Alaska|