But Mr Mackay, who awarded the contract to build the vessels for CalMac, said Scottish Government ministers were not to blame.
Speaking to STV News, Mr Mackay suggested procurement agency Caledonian Maritime Assets Limited (CMAL) had questions to answer over the fiasco.
The Glen Sannox and the as-yet-unnamed hull 802, are now more than three times over budget and years late. One is destined for the Arran route and the other will serve Skye, North Uist and Harris.
Their botched construction at the Ferguson Marine shipyard in Port Glasgow is the subject of a probe by the public audit committee which is due to report soon.
Mr Mackay reportedly told STV that the bid should arguably never have been put before ministers due to the lack of a “refund guarantee” to protect the taxpayer.
“I don’t know (if it should be called a scandal),” he said. “I don’t think so. I think that’s for other people to judge. It’s contract failure, certainly.
“I’ve said ‘contract failure’, because when we’ve had to deal with this issue in detail, it’s very important that I choose the right words. Other people can describe it as a scandal. That’s up to them.“It’s a scandalous state of affairs. Maybe we can agree on those wordings.”
He added: “I welcomed the award of the contract because it was good for Inverclyde, it was good for shipbuilding, it was good for the workers and we thought it would be good for the CalMac fleet.
“Ministers acted lawfully at the time and I certainly acted lawfully at the time. I do think CMAL has a case to answer, but I don’t want to prejudice that because there is an investigation under way.”