In 1915 Kitty Wix and her friends try to stop the arsonist who is terrifying their small New Zealand town. The show was voted NZ Best Children's Programme 1987. It was shown in UK, and released as the edited telemove Undercover Gang on VHS in Canada.
A visitor to the site reports that TVNZ have advised that 'The Fire Raiser' is not currently available for purchase. However, it might be on a list of kid-ult dramas from that time that the team at TVNZ are hoping to clear and release over the next couple of years. It is a complicated (and time consuming) process clearing such series so there is no specific release date at present, but it might be worth keeping an eye on tvnz.co.nz/dvd for any news.
Thanks to Giles Lavery for the information about the VHS release.