I'd be interested Colwyn, might give me a chance to show off to my Kiwi son-in-law!
I can't find my original source of the story but it appears in wikipedia (see below). In 1905 the NZ team toured the UK and in Cardiff, as elsewhere, they performed the haka at the start of the match. Wales had decided to reply and Welsh player Teddy Morgan led the crowd in singing
Hen Wlad Fy Nhadau. Here it gets a bit muddled. That song was
not officially the Welsh National Anthem; that was
God Bless the Prince of Wales a thoroughly forgettable piece imposed on Wales by the English I suspect. But
Hen Wlad Fy Nhadau had become established as the unofficial, popular national anthem - and it later became the official anthem. So its singing
at the start of a game is recognized as the first time an anthem had been used to open a game. After that it caught on and spread to many other sports and all around the world.
So there's your story for your Kiwi in-law.
https://en.wikipedia.org/wiki/Hen_Wlad_Fy_Nhadau