Qantec McWilliam Consulting Engineers in Australia was acquired by Opus in September 2007. The company was re-named Opus Qantec McWilliam.

Qantec McWilliam Consulting Engineers was formed after the merger of QANTEC and McWilliam Consulting Engineers in 2003.

QANTEC has a long history which stretches back to 1923 when it was founded by Lieutenant-Colonel W. J. Reinhold. Starting his days as a humble sole practitioner, W. J. Reinhold’s company quickly grew and over the years saw a variety of name changes and acquisitions over the next eighty years.

In 1989, the then Reinhold Engineering Consultants Pty Ltd merged with D. L. Stanaway and Associates, to form Reinhold and Stanaway Pty Ltd. By this time, the company had a number of offices located throughout Queensland and the Northern Territory.

1995 saw another change, with Reinhold and Stanaway Pty Ltd joining forces with Hughes Truman and Ludlow to form HTL Reinhold Pty Ltd, which would later be known as Hughes Truman Reinhold Pty Ltd. This expanded the company to include offices in New South Wales and the Australian Capital Territory. In 1996, the company also acquired Clarendon Consulting Engineers.

After several years of successful business, a group comprised of Dennis Sheehan, Col Pascoe, Pat Hogan, Neil Weller and Austin Grillmeier acquired the offices owned by Hughes Truman Reinhold Pty Ltd located within the QLD/NT region, and formed their own company, QANTEC, on the 1^st of September 2003.

McWilliam Consulting Engineers was founded in Brisbane in 1924 by Russell John McWilliam, a graduate of The University of Queensland. In 1959 the name was changed to RJ McWIlliam and Partners when four new partners joined the business.

Offices were established in Toowoomba and Townsville in 1969 and the Government's initiative to improve health services throughout Queensland prompted the opening of an office in Townsville.

The Southport office was opened on the Gold Coast in 1970 and in 1972 the partnership became a company known as McWilliam and Partners Pty Ltd.  Another office was opened in Maroochydore on the Sunshine Coast In 1979.

The company was renamed in 1987 to McWilliam Consulting Engineers Pty Ltd.