The Government and the Auckland Council remain divided over the need for a $2.4 billion rail tunnel through the central city.
The council on Thursday released a 700-page report which it says argues a compelling case for the tunnel, but Transport Minister Gerry Brownlee says it falls short.
The report responds to a Government request made in 2011 to weigh up alternatives to the council's hopes to build the rail tunnel.
It says other options using buses won't help in the longer term. One alternative - a tunnel for buses - would cost almost as much, require a lot of land and reach capacity perhaps in 13 years.
The report says the rail tunnel is the only option catering for city centre growth beyond 2041. Without it, traffic speeds will slow and growth opportunities will be stunted.
Mayor Len Brown says officials from Auckland and the Government worked on the report and agree on its major findings. He says it underlines the need for what is his own top priority.
"Clearly, I have an aspiration around timeframe - I want it done tomorrow. But we have to work with a government that is constrained in its own funding envelope.
"My hope is with this report now in front of the minister and the Government that the case is increasingly a case of about not whether, but when."
Mr Brown says he is disappointed that government officials question the quality of the new report, despite their having been involved in the work for more than a year.
However, Transport Minister Gerry Brownlee says the case for the tunnel is no stronger and calls the report a valiant attempt to make it stack up. He says he takes issue with some of the growth forecasts.
The Auckland Council has begun planning work, even though funding is uncertain.