This means approximately 83 turbines could be used, with the blades likely to be manufactured at MHI-Vestas' Isle of Wight factory.
It is believed the V164 is on the preferred supplier list for Dong's project, which was approved by the UK government in November.
Dong is yet to make a Final Investment Decision on the Walney Extension, but construction is planned to start by the end of this year.
When contacted by Windpower Monthly, Dong and MHI-Vestas refused to comment on the deal.
Last summer, Dong ordered 32 MHI-Vestas V164 turbines for its 258MW Burbo Bank Extension project, also in the Irish Sea.
A second contract between Dong and MHI-Vestas would mean that Siemens, which has a majority share in the UK offshore market, loses out on another project.
The last offshore UK contract won by Siemens was back in February 2014, when it was selected for the 450MW Neart Na Gaoithe project by Mainstream Renewables.