Personally, I think it's fine for people to cross-post stuff they think is relevant to the communities here. I'd prefer it over links to Reddit, because it lets people engage with the content here. I doubt posts in !newzealand@lemmit.online will get nearly as much attention as the ones on lemmy.nz. The Lemmit bot makes it pretty obvious that it's taken from Reddit. I think one guideline would be not to remove the bot's text from the post, nor the "cross-posted from" text that's generated when it's cross-posted here.

As for question posts, it would be a weird thing to cross-post. I guess if someone wanted to give credit to the OP on Reddit. I can't see that happening too often, unless it's a pretty broad question that generates discussion - in which case, it doesn't seem to matter to me if there's a separate discussion going on here.