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A New Zealand scheme to assess the sustainability of commercial buildings is deficient, says a British sustainable construction expert.

Dr Michael Sansom visited New Zealand to discuss the UK's construction standards as they related to sustainability issues.

Dr Sansom said New Zealand had developed something of a hybrid of UK and United States models, but had not included a rigorous life cycle analysis methodology for all building and construction products, such as glass, steel and timber.

New Zealand's green star rating system used different criteria and tests for different construction materials, so they were not weighted to a set of scientific and standard criteria to allow for comparisons, he said.

"There must be sound robust standards."

Dr Sansom said the UK had been working with green construction standards for more than 16 years, and New Zealand could learn from itssystem.

Green star ratings were increasingly important, and Britain had set a target of zero carbon domestic buildings by 2016 and commercial buildings by 2019.

Although established as a voluntary assessment and rating scheme, the Green Star was already becoming more important.

"If the United Kingdom experience is to be followed in New Zealand, it is likely that Green Star ratings will become mandatory for all new buildings in the future, and therefore it is imperative that the assessment criteria used are robust and comprehensive."

He accused the New Zealand system of "cherry picking" by assessing only a couple of issues on important materials.

"In the complex field of sustainable construction, this approach can lead to spurious design and material selection decisions that can actually have adverse impacts to those intended."

Dr Sansom, who was brought to New Zealand on behalf of the local steel construction industry, said that for instance, credits were given for use of steel with a recycled content.

"But by focusing on the recycled content of steel, the perverse situation could arrive whereby steel products with higher recycled contents were sourced overseas in countries that have less stringent environmental regulations and might not be committed to emissions reduction targets, such as Kyoto.

"This is in effect a distortion of the market and furthermore has a net negative impact on green house gas emissions since additional transport impacts are incurred."

Steel was not alone in his "single-issue" criticism of Green Star NZ.

The timber industry was similarly being affected by imports from countries with poor environmental credentials particularly in terms of the preservatives used to treat the timber, he said.

The Green Building Council which established the rating system could also be seen as lacking the transparency of the UK system which was independent of construction industry interests.

Dr Sansom said the council was comprised of a grouping of paying member organisations from the New Zealand construction and property industries.

"There appears to be a lack of transparency in the governance of the council and its technical committees."

Green Building Council chief executive Jane Henley said the council worked with construction industry leaders and welcomed the involvement of all members, as well as a number of non-members from government and research organisations, to ensure it was transparent.

The UK system had received millions in government funding to define a life cycle assessment (LCA) methodology, she said.

"New Zealand is yet to define its own LCA methodology. Until this has been done and accepted by the industry it will be very difficult to use LCA data to compare materials and implement an LCA type credit in Green Star rating tools."