American company Peabody Energy reduced its offer to buy each share of Macarthur Coal at $15 a share from its previous offer of $16 a share. The proposal came in the midst of the growing disagreement on the Australian government's proposed super-profits tax scheme.

The Queensland-based miner told its shareholders to take no action yet.

Since the government has announced a plan that could raise taxes on resources sector, Macarthur shares have fallen to $13.69. In April, Macarthur's share rose to as high as $16.71 as talks of acquisition and merger heightens.

The proposed tax measure would apply to a 40 per cent tax rate on super profits made from non-renewable mining resources from July 2012. It is expected to raise about $9 billion a year. Mining companies have expressed their opposition to the proposed tax scheme stressing that it would affect their operation and income and future investments. They warned of stepping up their campaign against it.

Treasurer Wayne Swan accused the mining bosses of "putting the gun" on national interest with their continued opposition to the proposed tax measure.

Macarthur is the world's biggest producer of pulverised coal injection (PCI) product. During the global financial crisis, the price of PCI coal plunged as low as 50 per cent; but with world growth bouncing back and on red-hot demand from China and India, coal prices have nearly doubled. Coal producers are after Macarthur because it is the world's biggest exporter of cheaper, cleaner fuel craved by steelmakers.