Judge Jane Farish found the company guilty on all nine charges laid against it for the November 2010 gas explosion at its Pike River coal mine that killed 29 men.
PRC made no contest to the charges.
"In this case, there were fundamental breaches of the Health and Safety in Employment Act which led to the unnecessary deaths of 29 men," Farish said in a written judgement.
The Greymouth judge said the breaches were "causative of the explosion and the subsequent deaths of the men who perished".
Farish’s full decision is expected to be released in two weeks.
The charges included such breaches as failing to take all steps to prevent employees coming to harm, with each charge carrying a maximum penalty of $NZ250,000 ($A205,000).
PRC is in receivership and owes secured creditors $20 million but could face up to $2 million of total penalties when it is sentenced on July 4 and 5.
The judgement follows a Royal Commission investigation last year, which found PRC failed to flag potential dangers as it tried to boost production at the South Island colliery.
The former Department of Labour – the mine regulator – brought the charges against PRC, Valley Longwall International Drilling and former PRC chief executive officer Peter Whittall.
Drilling company VLI Drilling was fined $NZ46,800 in October after admitting it failed to protect its own staff and Pike River mine staff.
A trial date for Whittall has yet to be set but to date he has denied all 12 charges laid against him under the Health and Safety in Employment Act.
Just last week, an independent report ordered to see if any New Zealand government employees were to blame for the mining disaster found a raft of systemic shortcomings but did not hold any individuals accountable.
“We have concluded that there were actions or (more often) inactions on the part of officials in both agencies that may have contributed to the tragedy,” the report stated.
The report was particularly scathing of the DOL, stating: “The fact that we do not recommend any employment action against any individual managers should not be seen as excusing the performance of the organisation as a whole.”
The investigators said DOL’s performance as a health and safety regulator was regularly “dysfunctional and ineffectual”