The money paid to the families of the men who died in the 2010 Pike River coal mine disaster has been described as "unlawful" and "unprincipled" in the Supreme Court in Wellington.
The final appeal, heard on Thursday by the court, is spearheaded by Sonya Rockhouse and Anna Osborne, who lost a son and husband respectively in the mine's explosion.
They are seeking to overturn the decision by Worksafe to drop 12 charges against Pike River boss Peter Whittall after he agreed to pay $3.41 million to the victims' families.
Nigel Hampton QC, appearing for Mrs Rockhouse and Mrs Osborne, said money was paid to curtail a prosecution.
"We see the money as a bargain made for charges to be dropped and it is unprecedented in New Zealand legal history.
"It is unprincipled - there was no judgement against the defendant and we contend the agreement reached is an unlawful one."
Mr Hampton said if the agreement was allowed to stand, it could have long-lasting effects on criminal and civil matters that come to court.
In response, Worksafe lawyer Aaron Martin said there was no "improper bargain" made.
"There was no meeting of minds to withdraw the charges.
"We have a prosecutor that has to turn his mind to a huge range of factors that will not see anyone receive financial gain."
Mr Martin said previous courts had ruled correctly in Worksafe's decision being a legal one.
The lawyers' submissions will conclude on Thursday, after which they will consider their decision.