The two-phase project is due to be fully completed by 2018 and will consist of both Siemens' 7MW turbine and MHI-Vestas' 8MW V164 offshore machine. Both manufacturers were named preferred suppliers for either phase earlier this year.
"I'm very pleased that we can now start construction of what will be the world's biggest offshore wind farm when completed. Building this offshore wind farm will bring us significantly closer to realising our strategy of having 6.5GW of installed capacity online by 2020," said Dong executive vice president Samuel Leupold.
Siemens will manufacture 47 direct drive turbines for the Walney Extension East phase. MHI-Vestas will supply 40 V164 turbines to the western phase.
It is the first confirmed order for the Siemens 7MW which was unveiled at the EWEA Offshore event in Copenhagen in March 2015.
Siemens said the Walney Extension deal with Dong was part of a framework agreement signed between the two companies in 2012.
MHI-Vestas' V164 will also be installed at Dong's nearby Burbo Bank Extension site, due to begin offshore construction next year.
Dong has split the project into two phases because the site has two distinct areas with differing features.
"This will allow us to optimise the site and create two separate phases of the project. 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," said a Dong spokesperson.
The Walney Extension was handed a final investment decision-enabling CfD subsidy by the UK government in April 2014 and was approved in November 2014.
Walney Extension is located 19km off the UK's west coast in the Irish Sea beside the 367.2MW Walney development, completed by Dong in 2012 and powered by 3.6MW Siemens turbines.
Danish vessels firm CT Offshore has won the contract to install the project's inter array cables across both phases.
