Definitely not a Wheeny grapefruit or Poorman orange / NZG. 100% certain on that. The zest/pith on NZG is not that bitter and I made marmalade with them pretty easily. They also tend to have a lot more of an orange flavour in them.
The other name is Goldfruit. Usually just called New Zealand Grapefruit these days though.
If you're saying tropical pomelos can be sour and bitter, well this was grown in upper North island which would be considered the limits of true grapefruit cultivation (for commercial grade anyway), so pomelo / true grapefruit would not be expected to be better than something from the islands. I just don't get why pomelo is so popular then if they're sour and bitter and probably can't even make marmalade with them either? Like NZG is also a bit sour and bitter, but way higher juice quantity and probably a lot sweeter too (and rind is perfect for marmalade). I get that this is probably a shitty pomelo so not comparable to a good one, but still...
The seeds look a bit wrong shape for citrumelo (too flat and rectangular, and fruits are usually seedless too), and I suspect the leaves of the tree wouldn't have led to it being identified as pomelo. However it's a good suggestion. Citrumelo is also not known to be grown in NZ. I guess we'll see with the seedlings what sort of leaves pop up.
Sorry the tree is many hours away, won't be visiting it for years at best
Very happy to answer any questions about Citrus in NZ.
NZ Lemonade is obviously very popular and widely grown. Meyer lemons are planted everywhere and must make up a solid 1/3 of backyard Citrus trees.
NZ Grapefruits too grow widely, almost down to 43S. Rest of the common Citrus are pretty similar to what is available overseas (lemons, limes, oranges, mandarins, tangors, tangelos, green finger lime starting to become more common)
The only other "unique" NZ Citrus would be the Maori lemon / Kaipara lemon, which is probably the exact same fruit as the Australian Bush Lemon (small, wrinkly yellow fruit, sometimes eaten whole, skin and all), which seems to be a sweet form of rough lemon.
There's a few recently released mandarin hybrids ('Bay Zest', 'Bay Sweetie', 'Bay Sunset' and 'Bay Gold') for home use, but I don't think they're particularly distinguishable from their parents really.