Stormont departments have unique legal power to take decisions in the absence of ministers, the Court of Appeal heard on Monday.
Senior judges were told Parliament anticipated periods of hiatus where Northern Ireland would have to be run without a functioning Executive in place.
The Department for Infrastructure is seeking to overturn a ruling that a permanent secretary unlawfully approved a £240m waste incinerator on the outskirts of north Belfast.
In May the High Court held that the senior official did not have the required legal authority to grant permission for the waste disposal facility at Hightown Quarry in Mallusk.
Appealing that verdict, senior counsel for the Department set out how ministerial posts have remained vacant since the devolved administration collapsed more than a year ago.
Tony McGleenan QC said: "In the 450 or so days that have passed since then, governance in Northern Ireland has continued to function because of the unique constitutional arrangements that operate in this jurisdiction, which allow for powers to be discharged by departments."
He argued that the legal right stemmed from the Northern Ireland Act - with no equivalent provisions for other parts of the UK.
"Thats a unique design feature, there's no analogue to be found in the Scotland Act or the Government of Wales Act."
Likening the departments at Stormont to corporate bodies, Mr McGleenan stressed civil servants were not making decisions as individual employees.
"We have departments discharging their statutory functions, there's an important conceptual difference," the barrister submitted.
"These are decisions of departments."
A 25-day period following the March 2017 Assembly elections triggered by the late Martin McGuinness's resignation as Deputy First Minister was cited as evidence of duties continuing in the absence of ministers.
The court was told social security decisions were taken on a daily basis to provide benefits for the most vulnerable and needy.
"This is an important constitutional case, which raises issues of significance for the governance of this jurisdiction," Mr McGleenan continued.
He insisted that departmental action has been restrained over the past 16 months, only taking significant decisions when there has been pressing public interest.
According to his case Westminster would not have empowered the Northern Ireland departments to spend £16bn of public money in that period if there were concerns about their legal authority.
The incinerator scheme had originally been turned down in 2015 by the then Environment Minister, Mark H Durkan.
But a consortium behind the project on behalf of six local councils, Arc21, was given permission after the Planning Appeals Commission recommended approval.
In September last year the Department said it was in the public interest for the waste management system to be built, describing it as being of strategic importance for the region.
The decision came months after the Stormont Executive collapsed in January 2017.
Up to 4,000 letters objecting to the incinerator were lodged, with residents listing concerns about the visual impact, light and noise pollution and health implications.
Judicial review proceedings were issued by Colin Buick, chairperson of community group NoArc21.
His lawyers contend that the decision lacked the direction and control of a minister required under legislation.
However, Attorney General John Larkin QC has backed the Department's case.
Intervening in the proceedings after raising issues of devolution, he contended that if properly followed, the legislation provides authority to civil servants.
"The question is can a permanent secretary do what he's now impugned for doing, and I submit that the answer is yes," he told the court.
David Scoffield QC, representing Mr Buick, argued that legislation has made clear a department's functions will "at all times" be subject to the direction and control of a minister.
"What that means is they must be under the watchful eye of a minister and liable to his or her intervention," he said.
Decision making without such control is democratically unaccountable and invalid, Mr Scoffield claimed.
In a reference to high-ranking civil servants, the barrister added: "No-one can punish Peter May or David Sterling at the polls at the next election if they don't like the decisions that have been taken."
The appeal continues