Fire-safety experts are issuing a fresh warning ahead of Guy Fawkes celebrations next week: Changing any characteristic of a firework is a criminal offence.
Wanganui's fire risk-management officer Shane Dudley said modified bombs in letter boxes still occurred occasionally, seven years after restrictions came into effect on the sale on sparklers to crack down on the practice.
A ban on skyrockets had been the biggest change Mr Dudley noticed. "Since they've outlawed them, there've been fewer and fewer instances [of fireworks abuse]," Mr Dudley said. "And certainly roman candles can cause fires, particularly with the hot, dry spring, although this year it's still fairly wet."
Special permits are needed for bonfires. These are issued by the Wanganui Fire Service after the proposed site has been inspected.
HazSubs runs approved handler courses for retailers of fireworks.
A HazSubs spokesman said it was fortunate that the age of modifying fireworks was largely a thing of the past.
"When I show a picture of a sparkler bomb, I look around and see if people are nodding and smiling," HazSubs operations manager Aaron Donald said. "Half of the people don't recognise [sparkler bombs] any more."
However, Mr Donald said, people would always need reminding.
"As part of my presentation, I bring to people's attention the facts around sparkler bombs and the danger they pose. Changing the characteristics of a firework in any way is against the Hazardous Substances and New Organism [HSNO] Act.
"If you're altering a firework in any way, that is an offence. So when you get these kids putting a bunch of sparklers together, turning it into a bomb, while they run the risk of maiming, they're committing an offence."
Mr Donald was hesitant to describe tying sparklers together as a criminal offence. "You might be splitting hairs, but if you're changing the characteristics in any way ... Clearly sparklers say on the packaging 'light only one sparkler at a time'. So if you strap them together, if that's resulted in damage, they could look at that as breaching the HSNO Act."
An improvised skyrocket put two elderly Auckland residents in hospital on October 4 after crashing through a window, filling the house with smoke.
Fireworks are permitted to be on sale for four days before, and including, Guy Fawkes Day. This begins tomorrow and ends at the close of business on November 5. Restrictions imposed in 2007 now mean no more than 50 sparklers can be purchased, mixed in with bulk packs.