Solid Energy planned to extend the Waikato mine to the north, adding 10 to 15 years to its life and increasing local employment from 145 to around 200 people.
The new agreement for the supply of coal to the steelworks at Glenbrook, near Auckland, is for five years.
"Concluding negotiations and securing the contract is very good news for Solid Energy and particularly the Huntly East Underground Mine," said Barry Bragg, chief operating officer for Solid Energy.
The coal will be supplied from both Rotowaro Mine and East Mine, but it was very important to the future of East Mine to secure this contract, he said.
Solid Energy plans to invest more than $100 million developing and extending East Mine over the next five years.
The major expenditure will be the construction of a mine ventilation shaft and the provision of mining equipment such as continuous miners, shuttle cars and conveyors.
Over the course of the now-concluded contract negotiations, Solid Energy sought and was granted resource consents from the Waikato District Council and Environment Waikato to extend the Huntly East Mine to the north.
New Zealand Steel employs approximately 140 people at the Glenbrook mill, which was specifically designed to produce steel from the North Island's West Coast iron sands and Huntly coal.
