The state's highest court has ordered an investigation into claims a juror in a sex crime trial posted comments on Facebook suggesting "sex predators" should be "put down" like dogs, a day before the jury found the defendant guilty of the crime.

The Office of the NSW Sheriff, which administers the jury system in the state, will also investigate whether a second juror in the District Court trial socialised regularly with a key prosecution witness.

The NSW Court of Appeal has ordered an investigation into potential juror misconduct. Credit:Peter Rae

The NSW Court of Appeal ordered the investigation on March 29 after lawyers for the defendant – who was convicted in December 2016 of sexual intercourse taking advantage of a person's cognitive impairment – applied for an order that the Sheriff investigate potential juror misconduct.

The court heard the juror who made the Facebook post, written a day before the guilty verdict, was related by marriage to a complainant in a separate trial who had alleged the defendant sexually assaulted her.