Don’t understand how it can happen unless you put it on
For sure it can happen naturally. I'd sure most people who speak a long time away from NZ, have their Kiwi accent soften for sure.
Don't forget Waine has English parents as well. So grew up in a household of Blighty voices.
Can happen in reverse. Years ago a mate from Uni, went to the UK, met a local lass (later married her) and took her back to the Hawkes Bay for a summer. From memory she worked in a freezing works or something whilst in the Bay. When they returned to Kent, apparently her parents insisted she have Elocution lessons as they weren't happy with the way she now spoke!! Eating her vowels and all that. Whenever he visited our London flat, he got a very hard time about it all.