NSW Premier Gladys Berejiklian speaks to the media during a tour of the new Martin Place metro station site in Sydney (Image: AAP/Joel Carrett)

Cronyism, protectionism and partisan politics are costing taxpayers billions as the cost of infrastructure mega-projects continue to rise -- to the point where Australia is one of the most expensive countries in the world to build infrastructure.

The Grattan Institute's second report on mega-projects details how politics drives the cost of big projects up, and results in a quarter of all projects having cost blowouts.

Three of the key factors in pushing up costs derive directly from our flawed political system and the partisan goals of politicians.