The contract extension is worth £40m a year and runs until 2018.
The programme of work will replace 420km of gas mains each year and complete 40,000 customer gas connections.
The contract extension follows Ofgem's price control review, which has set financial targets for each of the eight gas distribution networks.
Marc Boulter, managing director of Amec’s Transmission and Distribution Europe business, said: "Wales & West Utilities' decision to extend the contract is a reflection of the quality of service provided since the initial contract began. This successful alliance will remain focused on delivering WWU's business objectives and on meeting their mains replacement programme commitments."
Morrison Utility Services executive director Jim Arnold added: "We are very pleased to secure this five-year contract extension that will continue to see all parties working closely together to ensure this vital programme is delivered successfully across the Wales & West Utilities network.”