A new report reveals an upgrade of Northland's rail network will only be worthwhile if Northport's operations are expanded.

The Ministry of Transport business case examined the cost of upgrading the rail line between Auckland and Whangārei, reopening the mothballed tracks north to Moerewa and west to Dargaville, and constructing a new spur east to Northport.

It estimates the total cost at $1.3b over 40 years. Including $730m in the first four years during the construction phase and $3m for improvements and maintenance in the years after.