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The Montara oil spill which erupted on August 21 last year off the northwest Australian coast was the result of a mismanaged concrete pour six months earlier, according to evidence given during the first week of an official inquiry.

The H1 well, operated by the Australian arm of Thailand’s PTTEP, began leaking oil on 21 August, triggering a blaze on the oil rig which lasted for two days and caused Australia's worst offshore oil spill until it was successfully plugged on 1 November.

Noel Treasure, senior supervisor for PTTEP Australasia on the West Atlas rig in the Montara oilfield in the Timor Sea when the pour was made, told the Commission of Inquiry in Canberra that after cement being poured to form a blocking shoe flowed back to the surface, he decided it should be pumped back. Under questioning by counsel assisting the inquiry, he admitted miscalculating cement volumes during the pour.

“I shouldn't have pumped so much back and that's the top and bottom of it," Treasure said. "I wasn't out of my depth. I just made the mistakes."

The failed cement job and the absence of a pressure containment cap contributed to the well's blowout, the inquiry has heard. The operation’s onshore manager, Donald Millar, has told the inquiry he should have realised the problem existed from the daily drilling report.

"I didn't recognise the ramifications of that back flow and the re-injection of it," Millar said.

David Gouldin, operations manager for subcontractor Seadrill, told the inquiry that the issue of the missing pressure caps was “somewhat irrelevant” if the spill was caused by the failure of the cement shoe. Asked if the cementing was the root cause of the failure, Gouldin said: “There still are serious indications that was the case.”

Hearings for the inquiry will continue to the end of March.