The government has ordered additional reports into loading standards for balconies after the collapse of a balcony on Dunedin's Castle St which injured 18 people.

A government report released today said overloading was the cause of the collapse but noted some timber engineers had raised concerns about a practice called "notching'', which shortens joists.

Ministry of Business, Innovation and Employment Minister Dr Nick Smith delivered the report to a handful of media on Castle St this afternoon.

"The simple reason that this balcony collapsed was that nearly twice the number of people it was designed to support were on the balcony and many of those people were grouped at one end of it,'' Dr Smith said.

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The investigation found the balcony was built to standard and had a code of compliance certificate.

"The most valuable lesson from this accident is for people not to use residential balconies as grandstands. They are designed for ... about two people per square metre,'' he said.

Dr Smith had instructed government officials to undertake additional work into loading standards in respect of residential balconies that have a risk of being crowded, and into the building practice of "notching structural timber'' - a practice that meant the joists supporting the balcony were reduced in size from 200mm by 50mm to 150mm by 50mm on construction.

"This was quite within the code but is a building technique some timber engineers are concerned may create stress weakness,'' Dr Smith said.

An independent report commissioned by the Dunedin City Council and released earlier this month found the balcony was "critically overloaded''.