Speaking to media this morning, however, Collins said after a period of reflection "it's quite clear that the royal commission would normally be put in place if there was this sort of loss of life".
She said the cases before the courts will never get to the same level of work that a royal commission would.
Collins pointed out that there was a royal commission after the Pike River Mine explosions which led to significant changes in New Zealand's work safety rules.
"I find it very unusual that the Government [is] so opposed to a royal commission," she said.
But both Ardern and Workplace Relations and Safety Minister Michael Wood say this type of official inquiry has not been ruled out.
At the moment, WorkSafe is conducting an investigation which is looking into the events leading up to the eruption.
But Collins said that having WorkSafe acting as the regulator, the enforcer and the adviser is a "very complicated situation".
She said it would be like the Police being not only the investigators but also the judge and jury.
"It's pretty hard when you are all of those things."
Asked about an official inquiry into the events surrounding the eruption, Ardern said yesterday that she had not "absolutely ruled it out".
"There are still elements of the response to come," she said, during her post-Cabinet press conference.
"I understand that people recognise that the WorkSafe investigations are taking into account events leading up to the eruption.
"The coronial process does allow for some coverage of events post the eruption as well."
She said it would be remiss of the Government not to have those reports and that process completed before it makes any decision about whether "a gap that needs to be filled".
Meanwhile, Ardern said the Ministry of Business, Innovation and Employment (MBIE) has been called in to make sure WorkSafe had "fulfilled their obligations" in terms of its investigation.
Wood told media this morning that he is in the process of drawing up the terms of reference into MBIE's probe.
"I would term it a review – it's a look to make sure that we can have that assurance that WorkSafe have performed their functions in the way we would expect."
Asked if there was a possibility of an inquiry, he said: "We're just doing things one step at a time".
"If there were findings from this process that said we needed to do some further work, then we would take that into consideration."
