Stormont's Justice Department received less than half the extra Brexit funding it bid for.
Extra money was allocated for Northern Ireland as part of a formula for adjusting cash provided by the Treasury for public services.
Ulster Unionist Mike Nesbitt discovered the department, which oversees services like prisons and policing, sought almost £2 million for Brexit planning.
The amount allocated was £850,000, with £300,000 handed to the PSNI, an official letter to the Strangford Assembly member disclosed.
The money was used for staffing costs, and spending is under review to ensure the funding is put to best use, the response added.
Mr Nesbitt said: "It must be assumed that if the Department of Justice bid for £1.96 million then it is needed, so it is concerning to find they were allocated just £0.85 million, with an initial £0.3 million going to the PSNI.
"In his correspondence to me, permanent secretary Peter May makes clear that the bid included preparation costs for Brexit planning in the DoJ and PSNI.
"I will be writing to him to ask what the implications will be of them receiving such a small fraction of what they bid for."
Chief Constable George Hamilton has made it clear that justice could be dramatically affected by Brexit.
That includes the potential loss of the European Arrest Warrant, which could make extradition problematic, or even impossible at least in the short term, Mr Nesbitt added.
"So it is really concerning that the department didn't receive anything like what they asked for to assist in scoping out these issues."
According to a briefing to the voluntary sector, the Justice Department's priorities included participation in the European Arrest Warrant, maintaining the Common Travel Area of people between the UK and Ireland, and continued co-operation on cross-border criminal justice.
Another aim is ensuring effective reciprocal enforcement of civil and family court orders, allowing the placement of a child from Northern Ireland in another EU state.
A Justice Department statement said: "The department and PSNI received funding for activities relating to EU exit preparations this year.
"Funding to meet the future demands of delivering EU exit will be included as part of budget discussions going forward."