A SkyCity casino manager who promised a high-roller a $310,400 "goodwill payment" was justifiably fired, the Employment Relations Authority (ERA) has ruled.

Keith Hayashi, an operations manger at SkyCity in Auckland, mistakenly believed the baccarat player had been playing at the wrong table limit for nine days.

He promised her a $310,400 "goodwill payment", and while the error was caught before the payment was made, the player demanded and eventually obtained it based on his offer.

The details of the female player - understood to be a high value international client - are suppressed.

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In February of 2016 the unidentified woman played baccarat at the casino for nine days, in an area designed for significant international players.

Baccarat is a card game where players attempt to guess which of two hands adds up closest to nine. It is known to have a relatively low "house edge" and be favoured by wealthy gamblers from Asia, accounting for 91 per cent of Macau's casino income at one point. The house edge is the profit margin that casinos expect on games.

Hayashi was on a "sunrise" shift - 3.30am to noon - when he made the error.

He had been informed in a briefing that the table limit for a baccarat "any pair" side bet had been limited to $25,000 in a specific salon. He and several other workers believed the limit applied to a different room.

This small mix-up resulted in Hayashi approaching the player's table and assuming she had been playing to the wrong limit.

She had only been playing for a few minutes, but he was of the view that any compensation for the wrong maximum bet should apply retroactively to her entire nine days of play.

Hayashi then asked two other employees to calculate the amount owing to the client, coming up with a overpayment figure of $310,400.

Despite a payment of that size requiring authorisation from the chief executive, Hayashi advised the woman of the error and told her the payment would be forthcoming.

He then discussed the payment with his manager, who told him there would be no overpayment. He disagreed with this, proposing a "goodwill" payment to the high-value client. Hayashi did not tell his manager that he had already given the woman a figure.

Eventually he was convinced and returned to the woman to tell her there would be no extra payment.

She subsequently demanded the promised payment - and eventually received it.

Hayashi was dismissed in May, and took the decision to the ERA, arguing he was unfairly fired and seeking compensation.

ERA member Andrew Dallas found in favour of SkyCity.

"Mr Hayashi may have had the employer's best interests at heart, but SkyCity did not see it that way," he wrote in the decision.

"Unfortunately, what became, in effect, a personal crusade by Mr Hayashi to ensure that [the player], a significant client of the casino, was not disadvantaged by a highly unusual situation – a situation he substantially contributed to – has meant Mr Hayashi has lost his long career at SkyCity. This is, indeed, very unfortunate."

Hayashi had worked at SkyCity since 1995.