Investigations into the collapse of a silo full of 500 tonnes of milk in Southland have discovered cracks in four other silos at two Fonterra factories.
Cracks have been found in three more of the 15m silos next to the one that collapsed at the Fonterra's biggest factory in Edendale, Southland, last month.
No-one was hurt in that incident but production at the plant was hobbled for three weeks.
The fault has also been found at a huge stainless steel silo at the cooperative's Clandeboye plant near Timaru.
The cracks in the welds in the silos' bases were hidden behind insulation and cladding and were not picked up by routine testing.
All have been since been repaired and independently verified as safe.
Fonterra chief operating officer for global operations Robert Spurway said a wider investigation would look at how cracks could have been detected.
"We want to make absolutely certain that this sort of failure can't happen again."
Mr Spurway said a team, including engineers and metallurgists, was still investigating the Edendale collapse, and Fonterra was sure all its other thousands of silos around the country were safe.
"We have made a range of inspections across similar and different silos and found no other issues," he said.
Fonterra would not reveal the extent of the cracking because it was part of an insurance claim.
The cause of the weld cracks also remained a mystery five weeks on from what the company believes was the only collapse of its type in the world.
Investigators have put strain gauges on some silos at Edendale in their search for clues.
Mr Spurway said it was too early to know whether cracks at the Clandeboye structure meant the problem was not specific to the Edendale site.
He said it was unlikely poor quality steel played a part. "It's a very detailed investigation and all aspects of the design, construction and maintenance and servicing of the silo will be considered including the type of steel used - but that's not a factor that we expect is a problem."
Mercer Group designed and built all five silos
Christchurch-listed Mercer Group designed and built all five of the silos, among the many it has made for Fonterra and other agricultural companies.
The structure that collapsed at Edendale was seven years old but should have lasted decades.
Mercer issued a short disclosure notice to the Stock Exchange about this earlier this month.
It said in its annual report, out this week, that any impact from the collapse on Mercer was "unknown at this time".
Mercer Group chief executive Richard Rookes said today it was working with Fonterra and independent experts on the collapse investigation but had not yet found the cause.
Mr Rookes confirmed the cracked welding was part of that investigation.
He said the company had made about 20 silos for Fonterra and all the rest had been checked and cleared. The company had warned all its other customers.
The Dairy Workers Union assistant national secretary Angus McConnell said he had confidence in Fonterra putting priority on keeping workers safe.
Edendale's drying plant has been back up and running for a fortnight, in half the time that was expected.
Fonterra would not disclose the cost of the clean-up, disruption to production, repairs or investigation.