The negotiation skills of the Prime Minister, Julia Gillard, are not all they are cracked up to be, says the leader of the Greens, Christine Milne, as pressure mounts on the government to release details of the mining tax, which is believed to have raised little revenue so far.

''The Prime Minister is always cited as a great negotiator but I have pointed out it's not about whether you get an agreement, it's about whether the agreement you get actually delivers,'' Senator Milne said.

Under pressure … Julia Gillard.

The Greens and the Coalition are calling on the government to reveal how much it is receiving from the Minerals Resource Rent Tax.

The tax on the super profits of coal and iron ore miners was forecast to raise about $2 billion in 2012-13 but industry sources say the big three miners - BHP Billiton, Rio Tinto and Xstrata - don't expect to make any payments this week for the December quarter, repeating the reported outcome in the September quarter.