A climate change activist group has declared its support for the Greens' bid to get rail from Auckland's CBD to the North Shore by 2030.

The Green Party launched the campaign in Auckland yesterday as an alternative to the Government's proposal to have a road-only tunnel as a second harbour link.

The policy has been backed by Generation Zero.

Julie Anne Genter, the Greens' transport spokesperson, said a road-only tunnel will only add to congestion and will cost significantly more, without giving people on the North Shore "the benefits of access to the Auckland rail network".

Generation Zero's Auckland director Leroy Beckett said if there's going to be a new Waitemata harbour crossing then a rail option is desperately needed.

"I think there's wide support for the City Rail Link at the moment, and this is the natural extension of that," he said.

Beckett said pressure needs to be put on the Government for them to consider this.

"These projects take such a long time to get going, so you need to start campaigning for this stuff early to have any effect."