The project has involved changes to 13 IT systems and a review of HHG's disaster recovery arrangements.
HHG, which includes insurance companies Pearl and London Life, said the move to the new payment network would help improve security for the 25,000 payment transactions it sends to Bacs each month. The migration will also cut its running costs by automating administrative tasks.
HHG is using software from Eiger Systems running on an IBM server to transfer payroll information to the Bacs IP network. Computer Sciences Corporation runs the system.
The financial group has also set-up an identical server in a separate location as back-up.
Bacs is currently migrating 50,000 corporate customers on to its IP network. Users have until the end of 2005 to upgrade their Bacs connection software before the old network is switched off.
Bacs customers will also have to ensure that their payroll suppliers have updated their software so that payroll details can be transferred in a suitable format to Bacs' IP network.
In June, Bacs announced two connection services to its IP network, in partnership with BT.
Delivered over an extranet or a DSL broadband line, the services will give Bacs' corporate customers faster payments and a guarantee of 99.5% connection levels, according to Bacs and BT.
Under existing links to the Bacs IP network - over the internet or through a dial-up modem - companies may face delays in sending payments to Bacs due to problems in connecting to the internet.