Staff at NCT centres are 'firefighting' to try and deal with a major backlog.
That is according to transport consultant Conor Faughnan, who told The Pat Kenny Show this is down to a shortage of inspectors.
On Thursday the Dáil heard there are backlogs of up to six months.
Conor said the backlog has actually gotten worse in the last month or so.
"It hasn't got better, that's for sure," he said.
"In fact, on the stats, it looks as if it's got slightly worse since you and I last spoke.
"It's going to keep coming up because it's frustrating a lot of people. To simplify it: the NCT is short of inspectors."
Conor said the backlogs are all about manpower.
"They've been trying to recruit; they've been trying to tool up - but since the pandemic, they really haven't been able to fully tool up and get through the backlog," he said.
"We now have a waiting list of about 360,000 vehicles waiting to be tested."
He said when retests are taken into account, "that adds up to about four or five months worth of work still in the pipeline."
"The average wait is 25 days, but that average isn't really a useful figure - it varies quite a lot,” he said.
"To be fair to the NCT, they are doing their best to manage this situation... but they're firefighting.
"They have been firefighting now for a good six months and... they'll be disappointed themselves: the numbers have not come down in the last few months".
The transport consultant said suggestions of unavailability may be exaggerated.
"If you just go on to the NCT website now and book the next slot, the system might tell you it's not available until April or May, but the NCT are trying to dynamically manage this," he said.
"They do hold back slots, and what you'll find is, even as you're in the waiting list, earlier slots can come available for you."
He added: "The average is 25 days, and if your situation is urgent, they will try to facilitate you."