Rush-hour traffic in central Auckland will slow to walking pace if the central city rail tunnel isn't built, a confidential report warns.
The draft report by transport engineers Sinclair Knight Merz puts further pressure on the Government to back the project.
By 2021, most bus networks near and in the city centre will be at capacity or overloaded in terms of what can be provided on existing roads, the report says.
Private motor vehicle speeds will have halved from 16km/h in the morning peak to 8km/h.
The rail network will have reached the maximum number of services possible.
And by 2041, the bus network will be "significantly over capacity" and the average morning peak car speed in the city centre will be 5km/h.
Car journey times to the city centre from the west and south will increase by 30 to 50 per cent, adding an extra 30 minutes each way from the South Auckland growth area.
The report said there would be a 78c return for every $1 spent on the rail tunnel project - higher than other options.
A bus tunnel was not practical because too much land would have to be taken for it, it would reach its limits between 2025 and 2030 and it would cost up to $2.34 billion.
It would have a return of 28c to 36c for every dollar spent on it.
A surface bus solution was cheaper at $1.13 billion with a 34c to 50c return but had similar short-comings to the bus tunnel solution.
The report's 78c estimate of the rail tunnel's return was lower than last year's joint report by Auckland Council and Auckland Transport which predicted a return of $1.10 to $2.30 on every dollar invested.
Transport Minister Steven Joyce said that prediction was significantly overstated and claimed 40c was closer to the mark.
He instructed Mayor Len Brownto produce "a robust analysis of alternatives".
Sinclair Knight Merz was commissioned to produce a City Centre Future Access Study.
Auckland Transport is now analysing the draft version.
The draft also warns that by as early as 2021 growing congestion would "limit Auckland's potential growth" by increasing travel times for city centre workers and reducing efficiency for freight and commercial road users using the port, moving around the city centre, or passing through.
The growing congestion would also push employment out of the centre, reducing productivity and resulting in a less competitive economy.
By 2041, the report said, traffic jams would be keeping 15,200 employees and students out of the city centre and would reduce speeds for commuter, freight and commercial vehicles by 75 per cent.
Several city streets were carrying 80 to 100 buses an hour, resulting in "unstable flow and queuing".
Ultimately, main bus routes could have be be two-laned in both directions.
The report concluded underground rail was the only option with any capacity after 2041.
Naresh Kandpal, who buses from Glen Eden to the city centre and back every day said he was already noticing the traffic getting worse.
"I can see the wait is getting longer and longer every day for most of the buses," he said.
Once Government officials have responded to the draft, a final report is expected early next month.
A spokesman for Mr Brown said the Mayor had not yet seen the report.