The Ministry for Primary Industries has brought just four successful fish-dumping prosecutions since 2009, newly released information shows.

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The Ministry for Primary Industries has said there had been 28 such prosecutions since 2004.

But when RNZ asked for more details of those 28 cases, we were given documents showing just four prosecutions since 2009.

MPI has not gone any further back than 2009, saying it would take too much effort to collate that.

It sent the summary of facts for six cases. But two of those are not for fish dumping, and they are not of commercial operators: One is of an unemployed Tauranga man selling illegally to fish and chip shops, and another is a recreational fisherman taking loads of paua.

That leaves four dumping cases, with the most recent sentencing in March 2015. Others may be making their way through the court system now.

The four cases show various levels of dumping, including five tonne of snapper discarded in the Hauraki Gulf, and multiple dumping convictions for three Korean fishers on a big trawler whose dumping was large scale and endemic. In all four cases, the fishers forfeited their boats to the Crown.

The ministry's director-general Martyn Dunne has repeatedly claimed, in the face of claims his department was soft on illegal discarding, that they were prosecuting 300 cases and issuing over 3000 infringements a year. However, for dumping, these latest OIA figures show it is fewer than one a year.

The Ministry has previously downplayed the threat from dumping, while at the same time saying it targets it seriously.

There is a deadline of this Friday for public submissions on MPI's plans to bring in cameras on all boats by October 2018. It also wants electronic reporting and geospatial pinpointing of fishing boats by October next year.

Its own consultation document said there were "critical" risks to the quota management system from a lack of controls.

It said compliance efforts were being "severely" hindered by this, for instance, by having only 96 observers, which meant they went out on fewer than 5 percent of inshore boat trips, when anything under 20 percent was known to be inadequate.

It has not got the money to increase observers; covering 10 percent of the inshore and deep-sea industry would cost $14 million a year. Plus there was lots of resistance on boats to observers, including skippers removing sleeping berths deliberately.

Previously, Martyn Dunne and the Primary Industries Minister Nathan Guy have both dismissed a warning from one of fisheries most senior managers, Dave Turner, about fish dumping being systemic, and that wiping it out could also wipe out half the inshore fleet.

The Ministry's consultation document, however, said without onboard cameras "urgent" fisheries management issues would not be resolved. The costs are estimated at $5000-$18,000 per boat for installation, and $1000-2000 a year in servicing costs.