Sanford, the country's largest listed fishing group, will book a $5 million impairment charge on the sale of its last Pacific Tuna vessel when it reports half-year earnings on Thursday.

The Auckland-based company settled the sale of the San Nikunau having put that Pacific Tuna fleet up for sale when it decided to quit the business in September last year, Sanford said in a statement. Weak tuna prices led to "very limited interest" in those type of vessels, it said.

"To complete the exit from this unsustainable business, Sanford has taken an impairment of $5M to the book value of the San Nikunau," chief executive Volker Kuntzsch said.

"Whilst this is disappointing, we look forward to focusing on our strategy of value-add to our beautiful New Zealand seafood."

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Sanford reported $9.1 million of assets from discontinued operations as at September 30th, having already taken an impairment charge of $6 million on its pair of international purse seine vessels used for the Pacific Tuna business.

It sold another vessel in 2014 which had helped improve returns, but not enough to offset the higher access costs around the South Pacific and weak skipjack tuna prices.

The shares last traded at $5.65 and have increased 0.9 percent this year.