The Ministry for Primary Industries is refusing to release the final report into fish dumping that sparked an ongoing independent investigation.

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The draft report of Operation Achilles was leaked in May with video shot by MPI on five boats which showed them dumping catch off the South Canterbury coast three years ago.

The Ministry did not prosecute, but set up an inquiry to find out why not.

But it won't release the final Achilles report sought by RNZ under the Official Information Act, saying the investigative techniques it outlines must be kept under wraps or the Ministry might have to change what it does.

In a response to another OIA request, it said it had never had any plans to give fishing crews immunity from prosecution in return for allowing cameras on their boats.

RNZ had asked the Ministry, under the Official Information Act, for details of any such schemes.

But MPI said such documents did not exist because it had never formally considered doing this.

The lobby group Forest and Bird has questioned whether MPI has been giving fishermen assurances they won't be prosecuted if they help collect data.

MPI said in order to find out more about what fish was being discarded, it had set up an electronic reporting and monitoring system project.