Gareth Morgan says he will pull his money from Kiwibank if the agreed conditions of the purchase of his investment business aren't met.
The state-owned bank is buying Gareth Morgan Investments (GMI) including his KiwiSaver scheme.
Gareth Morgan Investments manages more than $1.5 billion of funds on behalf of 57,000 clients.
It will operate as a stand-alone entity within Kiwibank and Mr Morgan will remain on its investment strategy team.
However Mr Morgan says if Kiwibank reneges on its promise to honour GMI's current standards, such as being able to identify the stocks held in unit trusts, he will withdraw his money.
"But I don't think its an issue at all, because what Kiwibank is saying is that they adhere to those standards."
Gareth Morgan says he is selling his investment business because it has become too large for him to run.
He says the business has grown exponentially and its staff of 50 is more than he wants to manage.