Infrastructure Minister Nichola Mallon has ordered two independent reviews around how issues with lift equipment at MoT test centres led to tests for cars and light vehicles being suspended.
The Driver and Vehicle Agency (DVA) was forced to suspend all MoT testing for cars and light vehicles on Monday night due to safety concerns over cracks in vehicle inspection lifts.
The organisation warned replacing vehicle lifts at the 15 MoT centres could cost millions of pounds and take months.
A whistleblower claimed one lift collapsed at a centre and the issue was known as far back as November.
"I am very clear that this situation is not acceptable, I took up post two weeks ago and I have inherited this mess," said Nichola Mallon, speaking to the media during a visit to an MoT centre in Belfast.
"I have instructed the permanent secretary for the department for infrastructure to commission two reviews that will be overseen by me.
"The first review will be conducted by independent professional auditors - I want to understand exactly what happened here, who knew, what, when and what action was taken and the timelines involved.
"The second review, I have asked that an independent expert is appointed who can provide me with independent advice about the steps that now must be taken to ensure that we can get back to business as usual and safely in our MOT centres across Northern Ireland."
Meanwhile, the PSNI has said it has been in discussion with the DVA to understand the implications of the MoT contingency arrangements in the coming months.
Inspector Rosie Leech said: “DVA have announced a number of measures to help alleviate the situation for drivers regarding their MoT tests, which in turn should mean that insurance and tax cover for their vehicle is not affected.
"However, drivers should remember that the presence of an MoT exemption certificate is no guarantee of a vehicle’s ongoing roadworthiness and they should continue to ensure their vehicle is taxed, insured and complies with roadworthiness regulations.
“We would also advise that drivers in possession of an extension certificate, should keep it in their possession whilst driving.
“Police vehicles are exempt from MoT testing under the Motor Vehicle Testing Regulations (Northern Ireland) 2003. PSNI vehicles are regularly inspected in house by our specialist mechanics.”
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Earlier a Department for Infrastructure official admitted the near-closure of all Northern Ireland's MoT centres "came out of the blue".
Motorists have been advised not to attend scheduled tests and will be issued with exemption certificates for four months. Owners of four-year-old cars needing their first MoT and taxi drivers are being prioritised and they can not be exempted.
Heavy goods vehicles and buses are continuing to be tested because they do not involve use of lifts.
Julie Thompson from Stormont's Infrastructure Department said: "This has come out of the blue."
She told Stormont's infrastructure committee the lift suppliers were unable to provide "sufficient assurance" over the repairs that had been put in place.
"Yesterday came as a big shock," she added.
Ms Thompson said the lifts were first installed in 2011/12 and the department may have to buy new equipment. She said the parties involved need to learn lessons for the future to prevent the problem from recurring.
"When you are in a position whereby this has never happened before, this is very fluid and very recent," she added.
Signs of cracking were uncovered in 48 of the 55 vehicle lifts across the DVA's 15 centres, with the external contractor responsible for maintaining the lifts brought in to carry out repairs last week.
The problem was first known about in November.
Following further inspections this week, however, Paul Duffy the DVA chief executive said the contractor could not provide the assurances to the DVA as to the effectiveness of the ongoing repairs.
Speaking on BBC Radio Ulster's Good Morning Ulster programme, he said replacements and repairs could come as a significant cost to the public purse.
"We are looking at all options, there is a possibility they may need to be replaced. It would cost probably between £30,000 to £40,000 per lift, but we will need to go through some commercial discussions around that."
Mr Duffy said he could not give an exact time frame as to how long it would take to identify and replace affected lifts, but "preliminary enquiries" were taking place.
"Those are the issues we're grappling with today. There's is likely to be a lead-in time where lifts would have to be manufactured and then installed.
"We have started some enquiries, but we haven't got into the detail of how long [it would take to replace] 55 lifts."
Mr Duffy said it was likely to be a matter of weeks, but he "could not rule out" the process taking months.
"The most important thing is that we ensure our staff and customers are coming into a safe environment, so if it takes weeks or months to sort this issue out, it's important that we protect the safety of our staff.
If all 55 vehicle lifts had to be replaced at a cost of £40,000 each, this would amount to £2.2m.
All customers who have had tests cancelled will be issued a four-month exemption so they can continue to drive, however this does not apply to taxis and four-year-old vehicles.
Mr Duffy said that, while lanes at MoT centres where cars and light vehicle lifts are situated are not in operation, lanes for heavy goods vehicles are functioning. He said these lanes would be used to test taxis and four-year-old vehicles as a matter of priority.
Questions have also be raised regarding car insurance and if those with an MoT exemption who are involved in an accident will be adversely affected.
Malcolm Tarling of the Association of British Insurers advised affected drivers to notify there insurer if they have received an exemption certificate.
"Insurers will be pragmatic in these situations. Most motor policies do not specifically require you to have a valid MoT certificate, although all policies will expect you to keep your vehicle in a road-worthy condition and be on the road legally," he said.
"The key thing here is to talk to your insurer, let them know of these quite unusual circumstances and get a hold of an exemption certificate."
"Insurers are going to keep a very close eye on the situation and they are going to be guided by the advice the authorities in Northern Ireland are giving out."
Speaking on the Nolan Show on Monday, a whistleblower working at an MoT centre claimed one vehicle lift had collapsed.
The Department for Infrastructure (DfI) said that no such collapse had occurred in the last two weeks, however one lift suffered from an "hydraulic fault" last year which caused it to drop several centimeters.
The latest statistics from the DfI show that, from July to September last year, just over 290,000 tests were carried out by the DVA.
During the same period, the DVA received more than 273,000 applications for tests - a decrease of 4.1% from last year.