A video posted on Instagram shows a couple of young men lighting fires on the Hokitika Rugby League grounds in Wadeson Island.
The same youths who lit the fires then uploaded the footage to social media.
The video shows one of the young men lighting up the fires while the other one films and provides running commentary.
"Just 'cause why the f**k not?" the boy filming it says at one point.
Hokitika Rugby League Club (HRLC) Secretary, Kirsty Wilson, says she has no idea what the motivation behind the fires was.
"I'd expect it was sheer boredom," she told the Herald.
"They are not members of our club, and are not personally known by us," she added.
The club found the youths' names via members of the public, who alerted HRLC to the video on social media.
"Unfortunately I'm not sure what day they were on the grounds lighting the fires, but it must've been during this week sometime," Wilson says.
The club secretary says the ground is currently locked up ready to have the baleage cut - that is the biggest fundraiser to keep the club running throughout Winter.
Fortunately, the fires didn't catch. Had they done so, the club would have been in substantial financial trouble, and subscriptions paid by players would "most likely be increased".
"It would depend on the damage caused. We would need to replace goal posts, fencing posts etc and pray that the fire steered clear of our storage containers (which can be seen in the background of the video)," Wilson said.
Police are now investigating the incident.
"The video was sent to us by a member of the public, and also the Hokitika Police. We have not had any contact with the offenders, as we are leaving it with the police to deal with. However, we have been contacted by the local Fire Department who will be sending a Fenz official to pay them a visit."
The club posted the video on Facebook, where it has been viewed more than 5,300 times in under 24 hours.
"What on earth possesses kids to behave like this, especially after what Aussie have just been through?!" West Coast Rugby League Schoolboys said when they shared the video.