Former Reserve Bank governor Bernie Fraser says Julia Gillard struck a "sellout" deal on the mining tax but Tony Abbott has rocks in his head for wanting to rescind it.

Mr Fraser, the head of the RBA from 1989 to 1996 and a former Treasury boss, said Ken Henry's resources super profits tax (RSPT) in its original form was too complicated.

"It was never going to fly ... because it wasn't understandable ... it was over-engineered in a way," Mr Fraser told ABC Television on Wednesday.

Ms Gillard brokered a deal with the big miners soon after being installed as prime minister.

The resulting minerals resources rent tax (MRRT) was a sellout, Mr Fraser said.