An extension to Solid Energy’s Huntly East Underground Mine is vital to the future of the Glenbrook steel plant, a combined Waikato District Council and Environment Waikato consents hearing was told today.

Auckland City has been told to expect a land designation application next month for a $1 billion central city "loop" rail tunnel between Britomart and Mt Eden via Karangahape Rd.

"We are working towards designating the CBD rail loop, and expect to come to your council next month with a notice of requirement," Auckland Regional Transport Authority chief executive Fergus Gammie told the city's transport committee yesterday.

The authority considers a tunnel running beneath Albert St and including underground train stops near Wellesley St and Karangahape Rd to be essential for increasing the capacity of Britomart, by turning it from a terminal into an open-ended station.

But to do that, it needs electric trains, for which funding has yet to be determined after the Government said it would not allow Auckland Regional Council to raise a 5c a litre road-fuel tax in one hit next year.

The rail planners have also been thrown off-track by Auckland City planning commissioners, who have allowed retail giant Westfield to build a 41-storey skyscraper on its downtown site above the proposed tunnel.

Four motorway and rail tunnels are being recommended as Auckland's main future harbour crossing for up to $4.1 billion.

Officials yesterday confirmed their preferred option for four deep "driven" tunnels between Takapuna's Esmonde Rd motorway interchange and the Auckland isthmus, running beneath the western side of Wynyard Wharf, as disclosed by the Herald last week.