Warwick Slow, 19, an employee of radio station X105, said he was turned away from the main gate about 7pm, so jumped a fence about 50 metres down the road and walked up the driveway at Premier House in Thorndon.
Carrying a packet of Sizzlers sausages and half a loaf of Budget bread, he made it to the barbecue and band area, where he mingled with guests for about three minutes while on his cellphone to shock jock Iain Stables and live on air, Slow told Radio Live.
Police noticed him after a few minutes and Slow was taken to a basement, questioned and searched.
Prince William had not yet arrived when Slow made his entrance to the party.
Slow told the police his appearance at the barbecue was "a joke" and "a nice thing to do for the prince", and he had been put up to it by Stables.
Police would contact him later today about what action would be taken, he said.
It was possible he would be charged with trespass, but he didn't want a criminal conviction, due to his age, he said.
It was the second incident of the day for police, after a man was arrested outside the new Supreme Court, which Prince William opened yesterday.
The man, who was protesting about the Family Court, was shouting obscenities near Prime Minister John Key and ignored police warnings to tone down his language, Inspector Greg Gilpin said.
Mr Gilpin said there was "a bit of a scuffle" as the man was arrested, and the protester received a cut to his face.
Prince William visited children at Wellington Hospital this morning with Prime Minister John Key and his wife Bronagh and Health Minister Tony Ryall.
He spent an hour speaking with about 30 child patients in Ward 18 and unveiled a plaque to commemorate his visit which would later hang in the children's ward.
