Wellington Water's chief executive says someone in the company had an inkling about a budget error

The water provider's admitted it made a 51 million dollar mistake - during consultation with regional councils over long term plans.

It's confirmed there will be an investigation on what happened.

Tonya Haskell says somebody raised the risk there was an error, three-to-four weeks ago - but its scale only became apparent late last week.

She says it's had a major impact on councils coming up so late - but she's doesn't want to sack people over it.