LABOR is being questioned over its claim it would reap a net $36 billion over 10 years after fulfilling its election promises and making changes to negative gearing and the capital gains tax.
Costings by the Parliamentary Budget Office show that changes similar to Labor's policies would raise $800 million a year less than Labor has touted.
The PBO found the estimates were of "low reliability due to uncertainty" around how housing investors might respond to the policy if implemented.
However, such uncertainty surrounds most budget forecasts of major policy changes, whether from the PBO, Treasury or private economists.
Asked yesterday on ABC Radio National about the discrepancies, Labor leader Bill Shorten refused to release the detail underpinning the maths.
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