Hi MAPS - that makes it all a lot clearer, thank you.
Some initial thoughts:
You mention the death reg entry, but do you have John's NZ death certificate? Might it name his parents or place of origin as an Australian d/c does? Have you checked obituaries in case there is mention of where he was from?
Have you searched Australia to see if there are any John Hills or Charles Hills who remained there and married or died? You may need help from the Australian board.
I would say that there is a very good chance that these Hills are yours and the same Hills that ended up in NZ. The occupation is right, the names and ages are right, the fact that they travelled without wife and daughter who later came to NZ all fits in well. How sure are you that the Charles Hill aged 12 who travelled on the Lord Delaval is not son of John? Did he also go to NZ? If not yours he could well have been a nephew or other relative, or perhaps it was a co-incidence and he was no relation.
I may be wrong but I think you may a better response to this on either the Australia or NZ boards.
I'm not sure how much luck you will have trying to find them embarking in the UK ... Maybe you should post the question on the NZ board to see if anyone can locate their arrival there? (I can't help with NZ immigration) I know there was quite a lot of too-ing and fro-ing from Australia to NZ and vice versa.
One last thought - in the 1841 census the only clue to origin is whether or not the person was born in the county. Was John living in the City of London in 1841? (or Middlesex?
fF you go ahead and post about your Hills on the Australia or NZ board, provide a link to this post to give some background to the story.
I will let you know if I come up with any other ideas.