A teenager who allegedly robbed a pizza delivery driver at knifepoint on the doorstep of a home in Adelaide's north has been arrested and charged.

Key points: Police said the victim was not physically injured

Police said the victim was not physically injured An 18-year-old resident of the unit has been charged

An 18-year-old resident of the unit has been charged Police said they found him hiding in bushes in the backyard

Police said the driver had been dropping off a delivery at a unit on Waite Street at Elizabeth Downs just before 10:30pm.

He was then allegedly threatened by an 18-year-old resident.

"[He] was armed with a knife, and the suspect stole the pizza and drink that had been ordered to the premises," Senior Constable Matt Brown told ABC Radio Adelaide.

Police patrols were called to the property shortly afterwards and allegedly found the stolen items inside the unit and the suspect hiding in bushes in the backyard.

"As you can guess, Ali, police were supremely confident of an arrest," Senior Constable Brown told host Ali Clarke.

The 18-year-old was charged with aggravated robbery and will appear in the Elizabeth Magistrates Court today.

Police said the victim was not physically injured.

Meanwhile, police are treating a house fire at nearby Elizabeth Park as suspicious.

Emergency services were called to the scene on Yorketown Road just after 8:30pm.

Police said the house had been vacant for several weeks and the fire caused major damage.

They have asked anyone with information, or who may have seen people or vehicles in the area before the blaze, to contact Crime Stoppers on 1800 333 000.