The rock will be volcanic andesite, sourced from one quarry inland from Opunake and another north of Raetihi. It was durable enough to withstand the strong wave action at the mole, Ross said.
The shellrock at nearby Waitahinga Quarry was considered, but didn't meet the minimum requirements of coastal engineers.
Mills-Albert's was "hands down" the best of the tenders, Ross said.
"We have full confidence in Mills-Albert to complete the delivery to high standards."
Horizons had to consider price and whether the tenderer would give work to local people, Māori and Pasifika and people displaced by Covid-19.
The tenderer must also uphold Tupua te Kawa, the values of the Whanganui River settlement - "the natural law and value system of Te Awa Tupua".
Mills-Albert is forming relationships with iwi and hapū at the places rock will come from to ensure they support it being taken.
The Kāpiti company already works with businesses in the Whanganui region.
"The collaboration with the three subcontractors reflects the collaborative way we are working on the project - a lot of people coming together for a positive outcome," Ross said.
The mole repair is part of the $50 million Te Pūwaha project to revitalise Whanganui's port. Repair of the moles is being undertaken by Horizons, and they must be repaired for the port to function.
They define the river mouth and make sure its depth is navigable by vessels. They also protect port infrastructure from flooding.
For more information on the project see www.whanganui.govt.nz/port.
