By Katie Scotcher and Russell Palmer for RNZ
Waka Kotahi produced two large illuminated red 'zero' symbols this year at a cost of nearly $5000 each, to promote its "Road to Zero" campaign.
Wood said he became aware of them in March and contacted the agency urging it to take care over the amount it was spending on them. His office also made further enquiries.
"And that did eventually come to light, that there were a number more that were in production at that time and could not be stopped. So there are five in total of them."
Discovering a total of about $30,000 had been spent on the five signs, he told RNZ he had written a letter directly to the agency's chair to emphasise his views.
"I reinforced my support for the Road to Zero programme but did ask more caution was exercised around this kind of expenditure and we just consider whether it's strictly necessary as we move forward.
"I provided my views to Waka Kotahi that while they make the spending decisions at arm's length from the minister, I do expect them to exercise a bit more caution in the future."
He said Waka Kotahi had not misled the ministry, and he did not want to discourage it from spending on the campaign overall.
The Road to Zero campaign aims to reduce deaths and injuries from vehicle crashes, in part by making smaller, cheaper improvements like median barriers to the nation's more dangerous roads rather than focusing solely on expensive new ones.
The money for the large props came from the $4.7m million fund set aside for the campaign.
"We're investing in $2.9 billion on Road to Zero over the next three years, so this expenditure was a very small proportion of that," Wood said.
"Most of the money goes into improved roads, improved traffic policing, and things that will keep us safe, and we do need to invest in a strong public campaign to change attitudes and behaviors and I think most people would understand that. But just as we go along, we need to make sure that it feels proportionate and appropriate and that's what I've asked Waka Kotahi to consider."
He said other public awareness efforts like the 'Ghost Chips' TV ads and anti-drink driving campaigns had been successful in the past, and he did not want to discourage agencies from trying to deliver on their core business.
