GST report may be out before by-elections

A report on how the GST should be changed will be released "within weeks" but Malcolm Turnbull won't say if it will be out before the upcoming by-elections.

The Productivity Commission has handed its report on the way the GST is calculated to Treasurer Scott Morrison, and the contents could be a factor in by-elections in Tasmania, South Australia and WA.

"It will be released within weeks not months, but it's a huge report ... I haven't had the opportunity to read it, it's over 400 pages I might add, it's quite a doorstopper," Mr Turnbull told reporters in Tasmania on Wednesday.