"It was an incendiary device. There were four thrown under a van parked there.
"It could have been fatal but fortunately it wasn't," he said.
Three of the devices, apparently fireworks, did not explode.
NZMCA members gained entry to the site on Department of Conservation reserve off Derby Street through a combination lock.
Mr Constable said as the association was preparing to open the site in July last year, there was some dialogue with "hoons" miffed at their exclusive use of the area.
A kiosk built on the site for members to register their stay had barely been erected when "the buggers burned it down".
"There is an element in Westport that doesn't want us here. I think it's the young fellas who like to hoon around in their cars."
The association had liaised with Mr Howard and the Westport police to improve security as result of the latest incident.
The matter would also go to the national board meeting in Invercargill this weekend.
Westport was about to be awarded the association's Motorhomes friendly status, but the incident put a pall on it, Mr Constable said.
Greymouth was the first town in the South Island to gain this, and with 40 others following since, Westport was due to be formally awarded the status at the end of this month.
In light of efforts to make Westport attractive to visitors, Mr Howard said today that the town did not want "a large amount of publicity" about the incident by "a number of stupid youths".
"It's very unfortunate when we're trying to put out the welcome mat," Mr Howard said.
The local community patrol in conjunction with police and the council had stepped up monitoring of the park.