In a crackdown over unsafe and, at times, inhumane labour practices, all foreign fishing boats must now be reflagged with the New Zealand flag.

The substantial law-change came into effect on Sunday, after a four-year transition period, and means all vessels operating in New Zealand waters must follow New Zealand law.

Primary Industries Minister Nathan Guy said the change would ensure fair standards for all fishing crews in New Zealand.

SUPPLIED The 38-year-old Korean ship, Oyang 70, sunk on August 18, 2010, killing six of its Indonesian crew.

Reports of crews being beaten and forced to work for minimal pay, and for days without rest, have not been uncommon in recent years.

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"Reflagging gives us full jurisdiction over areas like employment, health and safety conditions on vessels fishing in New Zealand's Exclusive Economic Zone.

"The reflagging is carried out by Maritime New Zealand and requires operators to ensure fishing vessels fully comply with our maritime rules and the Health and Safety at Work Act. It also requires the crew to have appropriate New Zealand-equivalent qualifications," Guy said.

Nine vessels so far had been reflagged, three were in the process of reflagging and could not fish in New Zealand waters until they had.

About nine had decided not to fish in New Zealand waters, Guy said.

The nine that had reflagged are from Japan, Korea, Ukraine and the Commonwealth of Dominica.

In August 2010, the 38-year-old Korean fishing boat Oyang 70 sank in calm conditions off the coast of Otago. Six men died, when the captain refused to cut loose an enormous 120-tonne catch, causing the ship to roll and sink as the haul was brought in.

In 2011, all 32 Indonesian crew on the Korean Oyang 75 walked off the ship alleging sexual and physical abuse. The ship would later face 26 charges of dumping fish.

Its sister ship, the Oyang 77, had eight charges of illegally dumping fish overboard laid against it. Both were owned by Korea's largest fishing company, Sajo Oyang.

The string of incidents prompted the Korean Government to send an inter-departmental delegation to New Zealand to investigate concerns with Korean-owned fishing ships.

A New Zealand joint ministerial inquiry in 2012 found Korean fishing charters were damaging New Zealand's international reputation.

Former Primary Industries Minister David Carter and former Labour Minister Kate Wilkinson brought in the new rules that also included Compulsory individual New Zealand bank accounts for crew members, observers on all foreign-owned fishing vessels and independent audits of charter parties to ensure crew visa requirements – including wages – were being adhered to.

The industry as a whole catches fish worth more than $650 million a year, the majority of it filling Maori iwi fishing quotas. The export industry is worth more than $1.5 billion a year.