The 1883 lower Thames St courthouse was closed in November 2011 after being identified as earthquake-prone.
It was reopened in October after a close to $900,000 refurbishment funded by the Waitaki District Council that took ownership of the building in 2016.
Mr Perkins, an Oamaru ward councillor who was first elected in 2010, said the figure of up to $6 million first identified after an engineering assessment carried out for the Ministry of Justice about seven years ago was ''wildly inaccurate'' and questioned the then National government's motives.
He said Engineering New Zealand, the industry's professional body, should investigate the process and the findings of the report into the building's structural integrity.
''In theory, engineers operate objectively at all times. They look at the science at all times and come to a conclusion.
''It smells to me that they made an assessment to the government of the day that worked to withdraw court services from Oamaru and didn't take a lot of care and time to get to that conclusion.
"It's not how engineers should conduct themselves. I question whether whoever did that report was doing so by the engineers' code of ethics.
"Engineers have enormous power; you need to know that they are not influenced by anything other that the science that has contributed to their assessment. I would like Engineering New Zealand to look at it and respond.''
Oamaru lawyer Bill Dean, who with Dunedin engineer Lou Robinson and a local surveyor assessed the building and the cost of strengthening it to 100% as being $300,000 to $350,000, agreed with Mr Perkins that the initial cost was excessive.
However, Mr Dean questioned the value of an investigation and said little would be gained from such a move.
''I don't know what Hugh's driving at. It's yesterday's news. If he wants to hang somebody out to dry, then that's a matter for him. I'm curious to know how wide he wants this investigation to go.
"Is he going to look at all of the other estimates done around the country at that time? I don't know what the point is. It's all done and dusted.''
In a statement, National Party Courts spokesman Chris Penk said any suggestion the party had influenced the initial assessment were off the mark.
''There was never a conspiracy to shut the courthouse down. It's vital that justice is served everywhere in New Zealand, including, of course, in regional towns just as much as in the cities.''
Speaking in 2014 at the opening of the town's now former temporary courthouse in Humber St, known as the ''porta-court'', former minister for courts Chester Borrows explained the process which led to the closure and the then government's decision not to spend money strengthening it.
''We couldn't justify investing millions of dollars in a court with a small and declining workload, which is used less than one day a week on average, and where another solution was available.''
