Steel-test certificates for a huge Auckland sludge tank have been rubbished by the country's watchdog for laboratories, which says they are not acceptable for any regulatory purpose.

The only lab test certificates that exist for the eight million-litre sludge tank built by Fletcher Construction for Watercare at Mangere two years ago have not been endorsed by the Chinese accreditation authority.

Steel importer H J Asmuss has not responded to questions about this issue.

International Accreditation NZ chief executive Llew Richards said unendorsed certificates did not cut it.

"Laboratories often do take short cuts, if somebody wants a quick test and obviously it's much cheaper, then that is what they supply. But for any purpose that is regulated, these test results that are not endorsed are simply not acceptable."

He said he had concerns about structural steel being imported without valid certification from accredited labs.

The government is aiming to tighten testing of steel mesh by October as one of more than 30 changes to the Building Code to safeguard quality during the construction boom.

By the end of the year it is expected there will be seven labs accredited to test mesh, and a proper regime to do it.

Just two labs were accredited before RNZ began inquiries into mesh in March.

Aside from mesh, however, the official line from the Ministry of Business, Innovation and Employment so far has been that there is no evidence of problems with structural steel, such as frames and trusses.

However, RNZ's ongoing inquiries have shed light on six projects that have had or now have problems with bad structural steel itself, or around the lack of verified testing to local standards.

International Accreditation New Zealand chief executive Llew Richards said a closer look was needed and he was concerned about products coming from overseas.

"There's a lot of steel product coming from overseas where there is no independent testing, there's no verification of the measurements that steel is meant to have in terms of structural strength or chemical composition or anything, and I think this is a much broader issue that probably does need to be looked at as well."

The chemical composition of the failed steel at the Huntly Bypass on the Waikato Expressway was found to be under the requirements for seismic steel in bridges, after tests in New Zealand that contradicted the results of testing done in China.

It is understood that the contractors Fulton Hogan and HEB Construction took the steel to at least two other local testing labs, after it failed the first tests.

The Transport Agency said yesterday that the reinforcing steel used in a revised design for the Huntly bridge piers has been sourced within New Zealand and undergone independent testing and inspection

The revised design has been peer reviewed and certified by the consulting engineers.

Steel & Tube, which imported the bad steel, is in continuing contractual negotiations with an overseas supplier. The Transport Agency sent it back out into the market to get replacement steel.

The agency said its design and build contracts "place the burden of commercial risk and liability firmly on the shoulders of the contractor".

It maintains its quality assurance caught the bad steel; but it has refused to provide proof to back up its chronology around the testing, in the face of four industry sources who have told RNZ it was only caught because it buckled as they tried to drive the steel casings in.