A rival oil company removed petrol from the Lyttelton site beside Mobil's ruptured tank because it deemed the location too risky for storage.
Mobil is under investigation by Environment Canterbury (ECan) and Worksafe New Zealand after its secondary containment failed to hold 1.2 million litres of jet fuel.
The tank ruptured in a landslide on March 5. The tank's secondary containment, or bund, should have held all the jet fuel, but it had five holes and earlier repairs failed.
Z Energy said it had removed petrol from its site next door to Mobil and under the same cliff based on a cliff stability study by GNS Science.
The study found there was a low risk of the cliff collapsing but Z Energy general manager Rob Freeman and director of New Zealand Oil Services Limited (NZOSL), which manages the majority of the Lyttelton tank farm, said the more volatile substances were removed from the site. It became a diesel only site, which Freeman said reduced the risk further.
"Petrol has greater fumes and if there is a fire it is harder to contain, and it will potentially ignite easier so we took a view that that site where it was and considering the survey we would make it a diesel only site," Freeman said.
"It was not a requirement but it [removal] was the most appropriate thing to do."
Mobil said it carried out a post-quake site risk assessment.
A Mobil spokeswoman said its study deemed the risk of a neighbouring hillside collapse as "low".
"The one in 100 year weather event which initiated the landslide was not anticipated by anyone," she said.
The spokeswoman said it did "relatively minor" earthquake repairs to its bund, similar to those carried out by others in the tank farm.
However, ECan photos given to The Press show those repairs sprouted leaks, sending rivers of fuel out of the bund.
Freeman said NZSOL did hydro testing on all of its bunds after repairs were completed, repairs that included replacing some parts of the bund.
Water is put into the bunds and left for 24 hours to see how the bund performs.
"We are therefore very confident that our bunds will actually hold product," Freeman said.
It is a requirement that this is done every 10 years.
Mobil did not answer questions from The Press on whether it did hydro testing after the earthquake repairs.
The firm said its site was compliant with relevant legislation.
- The Press