Prime Minister Malcolm Turnbull's latest plan for the GST would create a new nightmare of complexity and controversy.

At least that was the advice offered up by Treasury when it was headed by Martin Parkinson – the man who now runs Mr Turnbull's own department.

Malcolm Turnbull and Colin Barnett on Barrow Island on Monday for a tour of the LNG project. Credit:Ray Strange

Mr Turnbull threw ailing West Australian Premier Colin Barnett a lifeline last week by pledging to introduce a GST "floor" – a threshold to ensure no state receives less than a certain share of revenue from the tax.

The commitment was short on detail but was welcomed by Mr Barnett, who is down in the polls and argues his state is being shortchanged.