Auckland's $1 billion-plus central rail tunnel project has been accorded a preferred route between Britomart and Mt Eden, although a business case has yet to be made for construction funding.

A $5 million investigation study for KiwiRail and the Auckland Regional Transport Authority has identified a 3km route rising to a height of 70m from the waterfront and including locations for three new underground stations.

Those organisations have lost no time in adopting a recommendation from consultants, who narrowed down the route from three short-listed options described to the Auckland Regional Council only last month.

Their choices of sites for the three stations are beneath Albert St - in the block between Victoria St and Wellesley St - as well as under Karangahape Rd and Symonds St.

Symonds St was a late contender, but was chosen in a bid to bring as much of the business district as possible within 500m of the rail network and to boost economic development around what Auckland City has nominated as a future growth centre.

The southern end of the route is a hybrid of two short-listed options, aimed at minimising the distance the tunnel will have to stretch below Spaghetti Junction before joining the western railway line near the existing Mt Eden surface station.

Transport authority planning chief Peter Clark said yesterday that although the tunnel would never be allowed to put such an important motorway corridor at risk, care had been taken to avoid a route too close to structures with deep foundations such as the Upper Queen St bridge.

KiwiRail chairman Jim Bolger said the project still had a long way to go but it was vital to protect the route to ensure greater reach and flexibility for Auckland's passenger network.

He said the preferred route required fewer curves than the other options, meaning lower costs for a boring machine and better operational speeds offering greater fuel economy for electric trains.

The route has the added advantage of running beneath public roads rather than private property for 2.4km.

Mr Clark said an estimated cost range of $1 billion to $1.5 billion had yet to be narrowed as part of a business case to be developing in association with concept design work over the rest of this year, before KiwiRail decides whether to seek a notice of requirement to protect the route.

But he said a benchmark should be the new A$2.3 billion ($2.98 billion) Epping to Chatswood underground railway in northern Sydney.

Authority strategic transport planning manager Matthew Rednall said the inner-city rail loop which an Auckland tunnel would create would build on the $1 billion rail electrification project and prove "transformational" to the development of the wider region as well as the central business district.

He said an expected 15-minute time saving offered by a more direct route for western line passengers than through Newmarket was likely to take pressure off bus services which faced increasing congestion with other surface traffic.

About 44,500 people work within 500m of the proposed Aotea Station, which the authority and KiwiRail envisage as a 170m tube about 10m to 15m beneath Albert St.

Given that the area is zoned for up to 80,000 workers and 11,500 residents, planners expect the station to be busier than Britomart.

Mr Clark said the Karangahape Rd and Symonds St sites, both more than 20m underground, each had 7000 people working within a 500m radius and were zoned for more than double that number.

He said the proposed Karangahape Station, situated beneath a ridge at the intersection with Pitt St, would be within easy reach of Ponsonby Rd.

Newton Station in Symonds St could offer entrances at all four points of the intersection with Khyber Pass Rd.

Although Transport Minister Steven Joyce said in October that he remained far from convinced about the need for a tunnel, the project is strongly supported by both leading Super City mayoral candidates, Auckland City Mayor John Banks and Manukau Mayor Len Brown.