Greens mining tax policy numbers questioned

Australian Mining reports:
 The costings of the Greens’ mining tax proposal for the election have been deemed to be able to be priced at “very low” reliability.
The party’s mining tax proposal is very similar to Kevin Rudd’s original controversial mining tax design. 
But the independent Parliamentary Budget Office, which was set up collaboratively by the Greens and Labor, cautioned the Greens’ predictions the mining tax will generate $20.8 billion are “extremely sensitive” to issues such as commodity prices fluctuations and the exchange rate. 
It also said there was no solid tax data that back the estimates, nor was there any solid indication on how taxpayers would receive the policy.
Read More at Australian Mining (link)