The prime minister doesn't believe a royal commission into the Whakaari / White Island eruption is necessary.
But National Party leader Judith Collins says if one was already underway, it could have provided additional evidence for the courts.
Twenty-two people died as a result of the eruption almost a year ago, and WorkSafe has been leading an investigation since then. Yesterday it announced its findings and laid criminal charges against 10 organisations and three individuals.
WorkSafe says this has been its most complex investigation ever.
Jacinda Ardern says both the health and safety regulator and the Coroner are investigating and a royal commission would have little to add.
"It does look like it could very much duplicate work that's already happening,'' she says.
Collins promised during the election campaign that a royal commission would be held. She has somewhat walked that back, saying it was now before the courts.
"If a royal commission had been started at the very start, which is probably when it would have been better, then that evidence could have possibly been used in the court case.''
East Coast MP Kiri Allan says the charges announced by WorkSafe yesterday have brought some relief to the community.
"There's been a sense that prosecutions may or may not be coming, so I guess it's the end of one part of this process, and to that extent I actually think there's some relief."
Allan says the community will be relieved the charges weren't laid any closer to the anniversary next Wednesday.
"This one year anniversary is important for all of us. We knew this was inevitably going to be a part of this period of time for everybody," she says.
"It's given some finality to this part of the process.''