An Austrian government expert slammed Thursday a toxic red mud reservoir bordering the Danube in northern Hungary as a "high risk site," questioning the stability of the dyke.

The bottom of the reservoir in Almasfuzito was water-permeable, allowing hazardous substances to flow into the Danube, Karl Lorber, a member of the Austrian government's Environmental Advisory Board, told a press conference organised by environmental pressure group Greenpeace.

"(The dam) was built with very modest technology... so it is no surprise that it is a high risk site now."

"Its rehabilitation is a must," he said, adding that the construction did not fulfill current environmental requirements.

"If red mud is at the base of the landfill, the dam will just slide into the Danube" as red mud turns into liquid under pressure, warned Lorber, who is also head of the institute for waste management at Austria's Leoben University.

The expert also noted the contamination risk from caustic red mud particles carried by the wind, which people might breathe in.

That the company operating the site "acquired a licence authorising it to deposit 161 different kinds of hazardous waste is the biggest risk," on top of concerns over a breach in the dyke, he added.

The licence, issued in April 2010, "is illegitimate on several counts," legal expert Csaba Kiss also told journalists.

In late September, Greenpeace staged a protest at the reservoir demanding its immediate closure due to the instability of the dyke.

A year ago, 10 people died and a state of emergency was declared when a similar red mud reservoir burst on October 4 at an alumina plant in Ajka, western Hungary, sending a million cubic metres (40 million cubic feet) of toxic mud cascading into surrounding villages.