A top lawyer working with the plaintiffs in a large-scale banking class action is calling for a formal investigation into the 'serious civil service process failures' by MBIE.

Rachael Reed, KC, has outlined her objections in a letter to Public Service Commissioner Sir Brian Roche and Attorney-General Judith Collins, voicing concerns with the Reserve Bank's risk assessment.

She says the $12.9 billion figure is 'fantastical' and says it doesn't have a grasp of reality.

"It just doesn't make sense on that first fundamental level."

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