Problems have been detected in the concrete base of what will be Auckland's tallest residential building.

The 52-storey Seascape apartment building in Customs St East house some 221 apartments and will be second only to the Sky Tower for height.

Construction was stopped for a few days when it was discovered that some of the concrete poured for the foundations was hollow in places and in need of repair.

none The 52-storey Seascape highrise in Customs St East, as designed by Peddle Thorp.

But China Construction New Zealand project director Greg Douglas said it was the problem that happened regularly on sites and it was nearly fixed.

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After chipping away the bad concrete, "you pour the product in, it goes back to full strength. It's just the process. You really don't know what you're going to find until you exacavate."

The Seascape differs from many projects in that it is foregoing piles for a technique called a diaphragm wall, a perimeter of poured concrete holes connected together.

Douglas said using a diaphragm wall was "more expensive, takes a bit longer but it's waterproof."

"We're very close to the sea here, and we're five storeys down so that's a long way down. The only other area like that around here is Britomart rail line."

Further down, there was rock which was a much better material for foundations.

PEDDLE THORP All up, the planned tower will house 221 apartments and studios.

The skyscraper is also awaiting approval from Auckland Council for its steel, which has insisted on a tougher testing regime after quality issues with imported steel.

About 1800 tonnes of steel had already been approved for temporary structures, and a building consent for the basement was likely within a couple of weeks. An application for consent for the super-structure would follow.

These were not huge impediments to a project which is aiming to ready by 2021, Douglas said. "It's a long process, it's a complex building and it's a very large building."

But while it was "not at all" a typical building project, it would be "an iconic building" when finished.

As well as apartments, the Seascape will have four floors of retail and restaurants at its base and a smaller tower, which will accommodate a new hotel.

An 11-storey office block with a heritage facade on the corner of Fort St will be incorporated into the design.