In February, MHI-Vestas was named as a preferred supplier for the project. The next stage will be for Dong to make a firm and unconditional order, usually after a Final Investment Decision.
MHI-Vestas would deliver up to 42 of its V164-8MW turbine to the project, 19 kilometres off England's northwest coast.
When MHI-Vestas was selected as a preferred supplier in Feburary, Dong revealed the 660MW Walney Extension project had been split in to two phases.
A Dong spokesperson said: "This, with the flexibility afforded to us through the phased Contract For Difference (CfD) agreements, enables us to exploit the opportunity to develop the project using different technology."
In March 2015, Dong named Siemens was the preferred supplier for the project's second phase. Siemens is set to deliver 47 of its 7MW turbine to the second 330MW phase.
The Walney Extension was handed a FID-enabling CfD subsidy by the UK government in April 2014.
This year the offshore joint venture has started to take its first orders. Including the Walney deal, MHI-Vestas has won firm orders, conditional orders or preferred supplier status for up to 2.3GW in 2015.
Earlier this week, MHI-Vestas was also named preferred supplier for Dong's 450MW Borkum Riffgrund 2 project off Germany, in the North Sea. The manufacturer will supply 56 V164 turbines.
Last month, the company also entered into a conditional agreement with developers Eneco and EDF Energy to supply the 970MW Navitus Bay, off the south of England. Also in May, MHI-Vestas secured a firm order for E.on's 400MW Rampion project, due east of Navitus Bay.
In January, MHI-Vestas entered into another conditional agreement with the developers behind the 165MW Belwind 2 project off Belgium.