The Mentalist

star Simon Baker was an unlikely insider to the plot to merge the ANZ and National banks, almost two months before the decision was made public.

The actor filmed a television advertisement promoting the merger in August, about six or seven weeks before it was revealed to New Zealanders.

ANZ representatives from New Zealand and Australia flew to Hollywood, at an undisclosed cost, to oversee filming of the promotion. The advertisement will air for the first time tonight but ANZ has given Herald on Sunday readers a sneak preview by posting it on YouTube this weekend.

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In the ad, the Australian-born star of The Mentalist talks about "two great names in banking coming together".

"The power of two - something Kiwis understand," Baker says in the 30-second clip.

The union representing the bank's staff said ANZ and National should have consulted with the public and their staff before the merger was decided.

First Union finance director Andrew Casidy said it was a bad look that the bank filmed a merger ad at the same time it publicly denied any decision had been made.

"We were publicly pushing the bank for a long time to come clean about what their plans were," Casidy said.

ANZ would not comment on the cost to film the ad in Los Angeles.