The contract is part of the `Efficiency and Growth Expansion Project' that will increase capacity at the refinery, in the state's south-west, from 3.5 million to 4.6 million tonnes a year.
Monadelphous will undertake structural, mechanical, piping, electrical and instrumentation works.
Monadelphous managing director Rob Velletri said that the proposed works continued a long association with the Worsley Alumina joint venture, which is 86 per cent-held by BHP Billiton.
Japan Alumina Associates and Japan's Sojitz Alumina hold the remaining 10 per cent and four per cent interest, respectively.
Velletri said the contract was a positive reinforcement of Monadelphous's "ability to seamlessly undertake large-scale, multidisciplinary engineering construction projects".
He said it also vindicated the company's decision to expand its maintenance and industrial services division in WA last year.
Monadelphous said pre-construction works had been approved and would commence immediately.
The contracted works are expected to be completed by the third quarter of calendar 2010.
BHP Billiton said in May that it expected first production at the expanded refinery in the first half of calendar 2011.
It said Worsley was one of the largest, lowest-cost and most efficient alumina refineries in the world, and the investment in further production capacity underlined Worsley Alumina's confidence in the future of the alumina market.
Shares in Monadelphous rose 35 cents, or 3.04 per cent, to close at $11.85.
Monadelphous had plunged an intraday low of $10.50 but rebounded, along with the broader market, on news that the Reserve Bank of Australia would cut the overnight cash rate by a full percentage point, the largest single easing in 16 years.
