The deal will kick off in January next year and has provisions for two extensions of two years each.
In total, the nine-year term would be valued up to $1 billion.
Downer will partner with LCC in the Logan Water Infrastructure Program Alliance to deliver planning, design, delivery and program management services for water, sewerage and treatment assets in the region.
Downer Chief Executive Officer, Grant Fenn, says the contract strengthens its position as a market leader in delivering complete water lifecycle solutions.
Shares in Downer EDI (ASX:DOW) are trading 0.6 per cent lower at $8.11.