The fishing industry says it supports observers on boats after revelations that some commercial vessels haven't let MPI staff on board.
The environmental lobby group Forest & Bird said it had obtained official data showing that 50 boats had refused observers over the past 18 months and that this illustrated the "industry is still plagued by rogue operators who appear to have something to hide".
Commercial vessels are required by law to allow observers on board but Forest & Bird say operators are using "manning limits" (effectively saying there is no room on board) to evade the rules.
Forest & Bird said that other operators were also using health and well-being concerns to not let observers on board.
"This calculated behaviour within the commercial fishing industry reveals an underbelly of lawlessness in a sector that has supposedly cleaned itself up," says Forest & Bird chief executive Kevin Hague.