A judge has rejected GNS Science's arguments for a split-off trial over alleged health and safety breaches at Whakaari / White Island.
Forty-seven people were on the crater when it erupted on 9 December 2019.
Twenty-two later died from extreme burns and blast injuries.
The Crown Research Institute is one of 12 parties facing a four-month trial this year - alongside tourism companies and the island's owners.
Yesterday lawyer Rachael Reed argued GNS was vulnerable to "collateral attacks" from other defendants, and sought a separate eight-day trial after a main trial for all others.
Rachael Reed KC told the Whakatāne District Court: "There is a real prejudice to GNS. It's not a normal prejudice ... It's entirely collateral, and would bring GNS into the main trial in a much greater way than it should because the reality is it's not involved in allegations in relation to the eruption, but it will be drawn into the mix."
She argued that GNS's charge was different to all of the others, because it related to helicopter trips to monitor the island - but none on the day of the eruption.
"No one else faces the same charge, and they're [GNS are] not charged together with any defendant," she said.
But this morning Judge Evangelos Thomas said: "The state of current court rosters in Auckland or Whakatāne means that a severed trial in Auckland would occur deep into 2024 and not at all in Whakatāne. After that, there is the coronial inquest. Everyone has already waited far too long. I must refuse the application."
He said "there are likely to be areas where the actions of GNS are important in any main trial" and that the communication between GNS and tourism operators, and vice versa, was central to the trial.
"It is inevitable that whether GNS failed in monitoring or advising risk, may be a live issue for those defendants."
He added: "GNS points out the cost and time that it would need to put to monitoring events at the main trial if it were part of it. It points to a lot of that work being unnecessary, given the amount of evidence that it says would not be relevant to the charge against it. GNS will have to monitor at a main trial anyway."
Yesterday, Kristy McDonald KC acting for WorkSafe, said "there is an incredible commonality of allegation and an enormous overlap in evidence" between GNS and the other accused.
In regards to the dates the charges related to, McDonald said: "It's the same risk. Different day, same risk. The charges against GNS, like the charges against the other defendant(s), allege a failure to adequately communicate risk."