The 17/18 Freight demand study, the first for five years, was released recently without fanfare. Here is a link to the report.
There are many interesting findings, too many to list here. But in a nutshell comparing 2017/18 with the previous report period in 2012, rail volume decreased 17% (partly due to the Kaikoura earthquake), and domestic sea freight volume increased 12%.
Overall billions of tonne/km increased 10.5% from 29.51 to 32.62.