The Transport Agency is taking a new look at how it measures the benefits of major projects.
NZTA believes Auckland's proposed downtown rail tunnel has only a quarter of the $1.3 billion of benefits claimed by Auckland Council - prompting criticism of the methods it uses. The tunnel would cost $2.4 billion.
The Government questions the claimed benefits, both the growth in rail patronage, and its economic impact in transforming Auckland.
Transport Minister Steven Joyce says Auckland has been tweaking the model used to measure the benefits.
Mayor Len Brown says a lot of the debate over benefits is crystal-ball gazing and government agencies have always underestimated Auckland's growth.
Part of the reason the Government attributed fewer benefits than Auckland to the project was the method it uses.
NZTA takes a narrower view than used in some other countries for measuring wider economic benefits.
But PricewaterhouseCooper director Chris Money says work done in Britain found that traditional ways of measuring, undervalued urban public transport.
Business commentator Rod Oram says that unlike the Transport Agency, the Auckland Council case measures economic growth from a more dynamic downtown area. He thinks the agency's focus is wrong.
NZTA has now put six research projects looking at new ways of measuring major transport projects, out for tender.
Auckland Council will next year prepare a more detailed case for funding the project.
More on the debate over Auckland's downtown rail tunnel can be heard on Insight on Sunday.