Australian gold miner Genesis Minerals launched a $5.6 billion Australian ($3.9 billion US) cash-and-stock bid for Vault Minerals on Monday, setting up a potential bidding contest in one of the world's biggest gold-producing countries.

Genesis' offer comes after Vault had already agreed to a merger with rival Regis Resources. The proposal values Vault at about $5.6 billion and represents a 14.5% premium to the existing deal, according to the companies.

Vault said its board considered the Genesis proposal a superior offer. Under the terms of its agreement, Regis has five business days to match or improve its bid. "Regis is considering its position," it said in a statement.

Genesis said the combined group would have a pro forma market capitalization of about $12.6 billion, which would make it Australia's third-largest listed gold miner. The merged company would produce 600,000 to 700,000 ounces of gold annually, Genesis said.

The company said the deal would create "a new Australian gold major" in Western Australia's Leonora and Laverton regions and unlock about $2 billion in post-tax synergies.

Genesis shareholders would own about 59.8% of the enlarged group, while Vault shareholders would hold the remaining 40.2%.

The possible bidding battle comes as accelerating gold prices fuel a wave of mergers among miners seeking to increase production, cut costs and attract global investors.

The recent wave of gold sector consolidation features Australia-based Northern Star Resources acquiring local peer De Grey Mining, alongside South Africa's Gold Fields taking over Australia-based Gold Road Resources.