On Easter Sunday I would think that some shops might be closed or have reduced trading hours.
Tourist attractions would probably still operate as normal. New Zealand does have some strict rules about Easter Sunday trading
The bus operator was Pacific Tourways. I think the tour operator could have been Leisure Tours New Zealand. It would be a good idea to try and obtain definite information about what would be open.
Otherwise I agree with you about cancelling.
Hopefully Royal wouldn’t operate the tour if they realize that most attractions would be closed - maybe not thinking that far in advance though.
Good luck and I hope you find some answers.