"Turn on a subscription, and receive a Free pair of $69 glasses after first automated order"
Limited FAQ around this, but there doesn't seem to be many restrictions on how you set up your subscription, so appear to be able to set this up and just order a box of lenses as a one off to claim this benefit.
You'll need to be logged into your Specasavers account to view this page:
https://www.specsavers.co.nz/my-account/easypay