The New Zealand Partnership-Based Temporary Visa Application form and guidance asks about visa refusals, whether you have ever been refused a visa/permit to visit, work, study or reside in any country, and to explain further (country, type, date(s), reason(s)). It does not direct you to attach copies of the document(s).
If you're unable to recollect those details, you would have to make a lost document request to Immigration, Refugees and Citizenship Canada (IRCC) through the Access to Information Act. To do so, however, you would need to engage a representative, as you are not in Canada.
Who can make a request under the Access to Information Act?
Canadian citizens, permanent residents and any individual present in Canada can make a request under the Access to Information Act. If you are none of the above, you can ask a representative, who is a Canadian citizen or permanent resident, to make a request for you, if you give that person written consent.