Premier Colin Barnett's hand-picked site for Perth's new 60,000-seat stadium contains junk including putrid organic waste, building rubble, coal ash, tyres, timber and even car bodies.
The revelation has sparked questions about whether costs will blow out if building conditions prove to be unsuitable.
The findings about the Burswood site - that it contains "uncontrolled fill" such as tyres, glass, rubber and tin cans to depths of up to 8m - are contained in tender documents calling for detailed geotechnical work associated with the stadium project.
In announcing last December that the stadium would be built on the northern nine holes of the Burswood golf course, Mr Barnett and Sports Minister Terry Waldron said there would be no "show stoppers" in preliminary geotechnical work to prevent the stadium being built there.
But the Government will not know for sure until this latest work, due by the end of June, is complete.
Environmental consultants, who declined to be named because they work on government projects, yesterday told _The West Australian _ that developing the site was likely to be complicated.
"It would be reasonable to expect that it will cost a lot and be an expensive site to develop," one consultant said.
"Investigating and working in contaminated sites is always a largely unknown exercise until works commence and the site has a long and chequered history relating to previous land uses."
Another consultant said: "We're doing some work on similar sites elsewhere in Perth.
"It all has to come out and be backfilled and compacted. Wherever you've got putrescibles in a landfill, that needs to be investigated and ruled out as a health risk."
Despite the revelation, Mr Waldron said yesterday the Government was confident about its assumptions that led to the announcement of the $700 million indicative cost for the stadium.
"The Burswood Peninsula has been subject to extensive historic geotechnical investigations associated with major residential and commercial developments and infrastructure projects," he said.
"The indicative estimate is based on the 2007 Perth Stadium Taskforce Report which allowed for measures to manage ground conditions."
That report, which put the cost of a Burswood stadium at $1.12 billion in 2007 dollars, said of Burswood that it was "a comparatively, $300 million-plus, more expensive site (than alternatives at Subiaco and East Perth)".
"(This is) due to the need to provide substantial transport infrastructure as well as the additional costs associated with site conditions (ie. reclaimed flood plain and site previously used as the Perth Municipal Rubbish Dump which included industrial as well as domestic landfill up until 1971)."
Opposition Leader Mark McGowan said the new revelations further buttressed Labor's case for the stadium to be built at Subiaco or East Perth.
"Dealing with all these issues will add to the cost of the stadium and a world-class stadium could be built much more cheaply elsewhere," he said.
Building stadiums on formerly contaminated land is not new.
There are dozens of examples around the world where stadiums have provided the impetus to clean up toxic sites.
The 2012 Olympic stadium in East London is built on land once used as a heavy chemicals storage facility. It cost the British taxpayer £13 million ($19.1 million) to remediate the site.
On a much larger scale, Sydney's Olympic precinct saw the remediation and restoration over 10 years of 160ha of contaminated land on the Parramatta River at a cost of $137 million.