The probable cause of the Montara oil spill was outlined by the operator of the Montara Platform, PTTEP, in its submission to the Montara Commission of Inquiry (MCI). According to the submission by PTTEP to the MCI, an initial uncontrolled hydrocarbon release (UHR) (containing approximately 40–60 bbl) occurred on the Montara Well Head Platform on the H1 Well at 05:30 on 21 August 2009.
This release subsided, although bubbles were seen and heard from the top of the conductor on the well. This was followed by another UHR at 07:23, of higher pressure and volume than the initial UHR, containing a mix of unignited oil and gas. Soon after, the rig was abandoned by all personnel, since safety was compromised due to the presence of unignited hydrocarbons. Soon after the second UHR at 07:23, PTTEP reported the UHR to AMSA, and control over the UHR was transferred to AMSA, in accordance with the provisions of NatPlan and NMOSCP. 3 Australian Maritime Safety Authority, Major Oil Spill: Montara Well Head Platform (2009)
Prior to transfer of responsibility to AMSA, PTTEP, as the relevant oil company, was the designated Combat Agency responsible for oil spills at its oil exploration rig