Siemens boss Joe Kaeser said the German conglomerate would announce by Monday whether it is to pull out of a project to supply rail infrastructure for a giant coal mine in Australia following massive environmental criticism.

Kaeser said Siemens, which supports the Paris climate agreement to curb carbon emissions, faced "a difficult decision" because it had a contractual obligation to deliver the signalling system for the planned mine in Queensland.

After holding talks with Germany's leading Fridays for Future activist Luisa Neubauer in Berlin on Friday, Kaeser said a final decision would be made by Monday.

The proposed Carmichael mine, owned by India's Adani group, has long been controversial but anger over the multi-billion-euro project has been fanned by Australia's catastrophic bushfire season.

Activists from Fridays for Future and Extinction Rebellion staged demos in dozen of German cities against the mine on Friday, including outside Siemens' Munich HQ.

A petition urging Siemens to quit the project drew some 60,000 signatures, while tens of thousands emailed Kaeser personally through the Fridays for Future website.

"Australia is on fire, we are in the midst of a climate crisis and burning coal contributes massively to the warming of the planet," it read.

"Stop supporting the destruction of our planet and our future!"

Siemens' contract to help develop the rail link that will transport coal extracted from the mine is only worth around 18 million euros ($20 million).

Protesters are urging Siemens to drop the Australian project
Protesters are urging Siemens to drop the Australian project DPA / Tobias Hase

But it would be a major symbolic victory for campaigners if the engineering giant bowed to its demands.

Siemens has set itself the goal of becoming carbon neutral by 2030.

In response to the mine backlash, it also claims that Siemens technologies helped reduce its customers' emissions by some 600 million tons in 2018, which it said was more than Australia's annual CO2 output.

The open-cut Carmichael mine is set to become operational next year and produce up to 27 million tonnes of coal annually.

The troubled project, which has been scaled down since it was first announced, has run into repeated delays caused by legal and regulatory hurdles, as well as funding problems.

But it has the staunch backing of Australian Prime Minister Scott Morrison.

Supporters say the mine will bring hundreds of much-needed jobs to rural Queensland in eastern Australia.

But conservationists say the project threatens local vulnerable species and means coal will have to be shipped from a port near the already-damaged Barrier Reef.

The world's largest coral reef system faces multiple threats to its survival, most notably rising sea temperatures caused by climate change, water pollution and coral-eating starfish.