The union representing workers at the Tiwai Point aluminium smelter and New Zealand Aluminium Smelters have reached a settlement of more than $20 million for years of underpayments.

Some 450 current and former employees will share the settlement, the E Tu union said in a statement.

E Tu and NZAS met last week to reach a deal, after the smelter owner's battle through the courts was turned down by the Supreme Court in April when it rejected an application to challenge a High Court ruling.

The dispute arose from workers being paid out for lieu days as if they normally worked eight hour days when their shifts were routinely 12 hours.

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Payments apply for all current staff employed before April 1, 2004, who worked those shifts in the past 25 years and will be based on calculations used by the 64 union members who challenged the smelter's payments. The deal also set a cut-off date for former employees of January 1, 2010.

The details are subject to a period of consultation which ends at the end of this week.