Hello Wendy,
As ever, Spades speaks wisely when he says a marriage certificate will help you find out about your grandmother's origin (age, place of birth, parents)
But the 1938 Stanley REES to Irene Margaret RICKARD marriage is classified as a non-historical marriage so won't be available to purchase on the BDM website (unless you have an iGovt ID login which I'm assuming you don't)
If you want to purchase the marriage certificate you'll need the folio number, which I can look-up for you at my local library tomorrow. Then you just phone BDM NZ on +64 (4) 463 9362 and you can order a printout of the marriage register.
A printout sometimes contains additional information not found on a certificate. And it's six dollars cheaper too :-)
The cost of a printout is NZ$20.40 so have a credit card ready.
Will get back to you with the folio number.
And don't feel obliged to make a purchase just because I visited the library. I'm heading that way tomorrow anyway.
Regards
Beg