It has emerged officials looked at more than just two or three alternatives when it came to South Island roading options after last year's Kaikoura earthquake.

Newly released papers show six possible inland and partial routes were looked at before the decision was made to repair and improve State Highway One.

Options considered were; a route via Molesworth station, another link via Waihopai, an inland route between Saint Arnaud and Hanmer Springs, and the current inland route through Lewis Pass.

Transport Minister Simon Bridges said in the end, the options away from the coast weren't realistic with cost just one of the factors involved.

"Much more than price was simply that they would take much longer to build, four to six years in a number of the cases, and also wouldn't add anything in terms of efficiency, safety, or resilience."

It has also been revealed proposals for KiwiRail to set up a Wellington to Lyttleton freight ferry service were dropped.

The idea was considered last year after the earthquake destroyed road and rail links on the South Island's east coast.

But the papers obtained under the Official Information Act show KiwiRail ended up recommending the idea not be adopted.

Simon Bridges said KiwiRail, like himself, saw a lot of potential for coastal shipping.

"The specified ferry would contain a capacity [that] didn't seem to be feasible on KiwiRail's advice, and look, I accepted that advice, given that there are other alternatives that we could use."