Correct phrase should be either "New Zealand is good at rugby" or Tha sgiobaidhean Shealainn Nuadh maith air rugbaidh - "New Zealand's teams are good at rugby"
Are you sure that's true in all dialects of English. I think some may allow the country name to be used as a plural grammatically like that.
From other sentences I've seen, I've got a feeling this is not wrong in Scottish English when referring to a sports team by their location.
Both will be accepted as correct, although I'd say 'are' is more common. Neither are wrong.
I'm an American and I would say "New Zealand IS good at rugby." New Zealand is counted as one entity, therefore it is singular.
That will be accepted here too.