Tuesday was the deadline for GVK to pay the outstanding money on its $US1.26 billion purchase of greenfield coal mines in central Queensland's untapped Galilee Basin.

But Fairfax Media understands that GVK was not going to meet the deadline prompting Hancock to extend the due date indefinitely.

The Indian conglomerate has put a $10 billion pricetag on the project, including about $US6 billion to develop port and rail, making it one of Australia's biggest coal projects.

Hancock picked the market well, selling its Alpha and Kevin's Corner coal projects to GVK in September, 2011, at the peak of the coal boom. At the time, thermal coal was fetching about $US150 a tonne, roughly double the $US72 a tonne it has sunk to today.

This dramatic price fall could have inspired Hancock Prospecting's leniency over GVK's deadline to cough up its final tranche.

It is understood the extension is open-ended for now, with a fresh deadline yet to be set.

GVK would not comment other than to say, "any such payments are a commercial matter and will be dealt with accordingly". Hancock Prospecting also declined to comment.

The original deal delivered GVK the Kevin's Corner mine and a 79 per cent stake in two neighbouring mines, Alpha and Alpha West, with Hancock Prospecting hanging on to the minority holding. Total reserves are put at eight billion tonnes.

A default on the sale is not in Hancock Prospecting's interests, particularly given recovering money from the GVK network of companies could be difficult. Hancock Prospecting also wants the project to get off the ground.

GVK, controlled by Indian entrepreneur GV Krishna Reddy, has seen its debt balloon since the deal was struck. Attempts to offload assets to repay debt have fallen flat.

It backed the Hancock mines purchase with a $US1.035 billion line of credit from Axis Bank of India.

But GVK is yet to secure project funding, put at up to $US10 billion. It is understood GVK would need to raise $US2 billion to $US3 billion in new equity before financial close in the Galilee.

GVK has been planning one of Australia's biggest coal projects , which would see it develop three coal mines in the Galilee, a 495-kilometre rail network and an export terminal at Abbot Point, with capacity for up to 60 million tonnes a year.

The project has suffered a string of delays. On striking the deal in September, 2011, GVK said "first coal production is expected in 2014". After a series of timetable blowouts, GVK last month flagged first coal for 2018.

However, a Queensland Treasury report suggests first coal exports of 1.6 million tonnes in 2019, increasing to 23 million tonnes by 2023.

In 2012, the Queensland government granted GVK exclusive rights for a rail corridor, and cut port access for several rival groups, including Clive Palmer's Waratah Coal.

Aurizon threw its weight behind the project early on, taking on GVK as a joint venture partner for the $6 billion rail and port underpinning its Galilee project. The deal would require Aurizon to upgrade part of its existing network to expand capacity.

The parties struck a non-binding MOU, initially set to be consummated by June, 2013. Aurizon last month again delayed a decision until early 2015.

Industry sources say GVK and Aurizon are pushing on with plans despite the near certainty of GVK missing Tuesday's payment.

The local GVK group is 90 per cent owned by the private GVK family group of companies, and 10 per cent by the Indian-listed conglomerate GVK Power & Infrastructure.

The listed GVK, which the GVK family majority owns, has a market capitalisation of about $US410 million, against net debt of $US3.3 billion.

The listed GVK group has provided loan guarantees against 49 per cent of the debt used to fund the $US1.26 billion deal with Hancock Prospecting.

If its Galilee projects gets up it will be one of the largest thermal coal operations in the world.

GVK has put cash costs of production for the project about $US55 a tonne but other estimates are as high as $US70 a tonne, energy adjusted and including port and rail charges.