Australian Greens leader Christine Milne says Prime Minister Julia Gillard isn't sticking to a promise of monthly updates on the mining tax because she's embarrassed it hasn't collected any revenue.

The minor party leader says Ms Gillard "hasn't been as transparent as she promised she would be" in a letter promising monthly revenue updates, written to then-Green leader Bob Brown when the tax legislation was before parliament.

"I think what we've got here is an embarrassment by both the prime minister and the treasurer because all that they promised with the mineral resources rent tax hasn't come to pass," Senator Milne told ABC radio on Monday.

Last week the government said it reveals combined mineral and petroleum resource rent tax receipts in its monthly financial statements.

But it said Australian Tax Office advice was that being more specific about the MRRT would breach privacy provisions.

"I think it's a convenient excuse in the context of an embarrassment whereby the MRRT has raised no money," Senator Milne said on Monday.

"The government can get round any problem like that if it chooses to do so."