A report due out today on Auckland's transport is expected to warn of gridlock for the city if it continues to rely on existing roads.

Mayor Len Brown will be releasing the Central City Future Access Study this afternoon.

The study has been done by independent engineers Sinclair Knight Merz.

Newstalk ZB believes they will repeat earlier warnings that without a rail tunnel, Auckland will need a four-lane bus way running out of the city by 2041.

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There is also expected to be criticism of the benefit-cost ratio used by the Government, which calculates the proposed tunnel would only return 78 cents economic benefit for every $1 spent.