Hillside, on the Yorke Peninsula, is one of the largest undeveloped copper projects in Australia with a resource of 2 million tonnes copper and 1.4 million ounces gold.

Under the agreement, Ausenco will deliver a project execution plan for the mine, a plant layout optimisation, bankable level details and a schedule.

It will also ensure design, layout and equipment selection accommodate a potential expansion of the project.

Rex is initially targeting a 6 million tonne per annum mine but would boost this to 8Mtpa in a second stage development.

The expansion hinges on coarse particle floatation systems.

Rex chief executive Richard Laufmann cited Ausenco's track record in copper project work as a key reason for the award.

The engineering firm previously delivered the Carrapateena copper project in South Australia in 2020.

"[Ausenco] will have an immediate and positive impact to our capabilities for the timely delivery of the Hillside Copper Project," Laufmann said.

"As medium-to-long-term ‘green' copper demand is championed across the globe, near-term copper production at Hillside has just taken a significant leap forward."

Hillside is located within the Moonta Subdomain of the Olympic copper and gold province, which also hosts BHP's Olympic Dam project.

Plant construction is expected to begin in early 2023 with first ore produced in 2025.

The processing facility will include a single stage semi-autogenous grinding mill with a conventional roughing circuit.

Rex plans to export copper concentrates from Port Adelaide, which has capacity to handle Hillside production.

In the second half of last year, Rex raised $50 million to pursue the project. It is looking to finance options to fund development.

"We remain very positive in relation to the project's economics and the longer-term supply-demand fundamentals for copper," Laufmann said.

"This is especially the case as the world marches confidently and irreversibly forward to a green electrification platform."

Hillside is estimated to have all-in sustaining costs of $1.60 per pound but assumed a copper price of US$3/lb and a gold price of US$1550 per ounce, delivering an internal rate of return post take of 16.2%.

Copper prices are trading at double this assumed price at the moment.