IBM has said it regrets the chaos caused by a computer fault on Sunday following suggestions from Air New Zealand that it may look for a new IT supplier.
The IT outage crashed airport check-in systems, as well as on-line bookings and call centre systems about 9.30am Sunday, affecting more than 10,000 passengers and throwing airports into disarray.
Most systems were restored about 1.30pm, but the passenger backlog did not start to clear until self check-in kiosks were up and running again about 3.30pm.
An internal email from Air NZ chief executive Rob Fyfe on Monday described IBM's service during the outage as "unacceptable" and "amateur".
He said he struggled "to recall a time where I have seen a supplier so slow to react to a catastrophic system failure such as this and so unwilling to accept responsibility and apologise to its client and its client's customers" during his working career.
"My expectations of IBM were far higher than the amateur results that were delivered yesterday, and I have been left with no option but to ask the IT team to review the full range of options available to us to ensure we have an IT supplier whom we have confidence in and one who understands and is fully committed to our business and the needs of our customers."
In a statement on Monday night, IBM said it regretted any inconvenience caused to its clients or their customers.
The company said the cause of the outage at the Newton data centre had not been fully determined, but the likely cause appeared to have been a failed oil pressure sensor on a backup generator.
"IBM's primary focus was to rapidly restore services to our clients, and in particular to Air New Zealand," it said.
"IBM immediately engaged a team of 32 local IT professionals supported by global colleagues and management to restore impacted client systems. Services to most clients were restored within an hour of the outage."
Air NZ outsourced its mainframe to IBM in 1997. Four years later, it also outsourced its mid-range systems to IBM.