Methane gas leaking from a closed landfill into neighbouring houses is a problem almost without precedent on a world scale, fire authorities have told a public meeting.
Residents of a the Brookland Greens housing estate at Cranbourne, south-east of Melbourne, have been urged to leave their homes because of the leak, which is creeping through the ground to the surface in an area beside the old tip, which has been capped.
Dangerous concentrations of methane have been detected in confined areas including the wall cavity of a house, under the kitchen sink of another and in stormwater pits around the estate.
At a public meeting before a rowdy crowd of more than 500, Country Fire Authority assistant chief fire officer Stephen Warrington said there had been very few similar incidents.
"Unfortunately for us, this is quite unique for Australia and most of the world. We don't know the ramifications of it," he told the meeting.
An earlier incident in England had revealed a 250-metre cordon should be drawn around an affected house to ensure the safety of all nearby.
Following that rule, much of the northern section of the estate was in the highest risk zone, he said.
He warned the crowd of the "very real risk" of explosion but said there were ways to reduce the danger if residents decided to stay.
"The only 100 per cent sure way to eliminate this risk from residents is for them to relocate from their homes," Mr Warrington said.
"Should you choose not to relocate, and I suspect that would be a large majority of people in this room... then there is a range of actions you can take on your property to increase your safety.
"We are not saying panic and run for the hills, but we cannot give you a gilt-edged guarantee. We can give you enough information to mitigate the risk."
Tips included adequately ventilating all confined spaces to allow a build-up of methane to escape, reporting gas build-up quickly and avoiding the use of ignition sources in confined spaces, Mr Warrington said.
Checks revealed no houses had dangerous levels of methane but the situation could change any time, Mr Warrington said.
More than 70 monitors have been installed in houses, with a further 300 ordered for the neighbourhood.
The Victorian government had allocated $3 million to buy new monitors and assist with building vents and other engineering solutions.
The local council, the City of Casey, originally rejected parts of the residential proposal by developer Peet Ltd, but the decision was overturned by the Victorian Civil and Administrative Tribunal in September 2004.
The president of the Cranbourne and District Residents Association, David Banner, told The Age: "Developers are getting their own way ... but once they've built it, once they've sold it and once they've moved on they've got the profit and the community and local council has to clean up the mess."
But in a statement, Peet Ltd said it had been advocating on behalf of residents for about two years, urging the council to improve management of the site to eliminate the risk of gas leaking.
Residents already angry that the meeting was hastily relocated because of a larger-than-expected turnout, were not satisfied.
Home owner Peter Turner said: "The Premier should have been here for a start. Peet should have been there. It was a waste of time."
Teresa La Paglia said: "Where is the machine to take home to make sure we are safe? Do we have to sleep with all the doors and the windows open?"
Police and Emergency Services Minister Bob Cameron said emergency grants of up to $1,067 each would be given to affected families.
Temporary accommodation and other support would also be provided as needed, he told parliament.
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