Well, it might not be the practice, but its the law, and (having lodged a request of my own specifically to generate an appealable refusal) I now have the Ombudsman's ruling to prove it. The key lines:

Further, the Ombudsman rejected the use of s9(2)(a) (privacy) and s9(2)(ba) (confidentiality) in this case, and noted that even if they had applied, they would have been trumped by the public interest. In my case, that means I get to know whether Key had communicated with Glucina about pony-tail pulling incident or victim (the answer to which, from the Ombudsman's ruling, appears to be "yes"). As for the original requester (who had complained as well), this should mean that they will be receiving the content of that communication shortly.
As for the supposed consequences on the relationship between Ministers and journalists, there's exactly zero public interest in protecting the ability of the powerful to smear using media proxies. And if Key is so ashamed of his contact with Rachel Glucina that he will blatantly ignore the law to avoid admitting it, maybe he shouldn't talk to her in the first place.