AMP said Salt's appointment followed a "comprehensive investment management search and evaluation process".
Salt was established in 2013 by former members of BT Funds Management, who went out on their own with a $650 million mandate from BT, which is owned by Australian banking giant Westpac.
The company, which specialises in Australasian equities and property investments, is owned by its staff, including managing directors Paul Harrison and Matthew Goodson.
Harrison said the new mandate would give Salt "critical mass".
"We're very happy with the position it puts us in," he said.
Harrison said Salt would not need to hire new staff and still had "headroom" to take on additional mandates.
AMP Capital head of distribution George Carter said Salt was selected because of its ability to align with AMP's investment philosophy.
"Being a core manager, Salt is an extremely good fit for AMP Capital and our clients," he said.
From next month, Salt will be the investment manager for AMP's NZ Shares Fund, Strategic NZ Shares Fund and Responsible Investment Leaders NZ Shares Fund, as well as the New Zealand equity component of the AMP Capital Income Generator Fund, AMP said.
Funds under management in those portfolios totalled around $690 million on February 28.