A Senate inquiry will examine to what extent the design of Labor's mining tax has contributed to a substantial shortfall in expected revenue.

The Australian Greens have successfully referred the Gillard government's minerals resource rent tax (MRRT) to an inquiry by the Senate's economics committee.

The committee has been asked to report back on May 6, a week before the federal budget is handed down.

The inquiry will examine the design of the tax and the extent to which it is responsible for the "mismatch between actual revenue and revenue projections".

In the first six months of its operation the MRRT has delivered just $126 million to government coffers, against a full-year forecast of $2 billion.

The committee will also look at the design process and the extent of involvement by Treasury and mining companies.

The MRRT evolved from negotiations Prime Minister Julia Gillard and Treasurer Wayne Swan had with the three big miners BHP Billiton, Rio Tinto and XStrata.

Greens leader Christine Milne said the inquiry was a "win for transparency."