Attention should be turned to bringing passenger rail to the South Island after the success of Auckland's City Rail Link, a train advocate says.

Save our Trains Southern has called for passenger rail from Amberley, north of Christchurch, To Ashburton in the south and between Christchurch, Dunedin and Invercargill.

The Canterbury plan would include Rolleston in the Selwyn district, the fastest growing town in the country.

Spokesman Dave MacPherson said the CRL had shown there was strong support for passenger rail when it was offered.

"It's actually time to start translating that support you see in pockets in the North Island, to the rest of the country. And particularly the South Island, where not a brass razoo has been spent on passenger rail by the government."

Macpherson felt the South Island was often forgotten about by politicians.

"What they haven't noticed is that the past two years, the South Island population has been growing at a faster rate than the North Island.

"So if you're looking to the future, you should be changing your thinking."

He said Save our Trains had spoken to South Island MPs about their proposal, and New Zealand First, Labour and the Green Party had supported their ideas in principle.

"They say they support us, and I believe they do. But it's not yet a priority for their parties, and I'd like it to be."

Macpherson estimated it would cost $112 million to buy the necessary trains, and said the tracks were already in place.

"It's not like building a new highway when you start from scratch. We've got a track here now."

That would be just over 10% the amount the government has spent on upgrading the Wairarapa and Manawatū lines.

That cost could be split between government, councils, and collected through fares, Macpherson said.

Transport Minister Chris Bishop said the government's current focus was on investing in the existing metro rail networks in Auckland and Wellington, which had suffered from years of under-investment in track maintenance.