A set of agreed facts tendered to Wollongong Local Court on Wednesday said Jackson Lamb arrived at the Westside Petroleum Service Station on the Princes Highway just after 4am on April 1 and approached the night counter, where he spoke with the male attendant.

"Can I get cash out? How much can I get out?" Lamb asked the man.

"No, we can't do cash out after hours," the victim responded.

Lamb then asked the man for a packet of Winfield Blues cigarettes, two Telstra SIM cards, a battery power bank and wireless headphones.

The victim put the items in a bag and told Lamb the total was $107.

Lamb said "I left my wallet at home, I don't have any money right now, can I come back and pay tomorrow?"

The victim replied "no, you have to pay now."

Lamb then put his hand through the window and while holding a knife, told the victim "I know you're a nice guy, you don't want any problems with me, I've been here before, I'll come back".

Lamb moved his hand around with the knife in it, causing the victim to feel frightened he was going to be hurt and prompting him to hand over the items.

Lamb stood around for a minute before walking off, but then returned to the counter and requested a lighter.

Lamb said "you're not calling the cops are you?"

The victim said he wasn't.

Lamb then asked the victim to write his number down, which the victim did, and Lamb walked off.

The victim then called his manager and told him what had occurred.

Meanwhile, Lamb returned to the service station an hour later, asking if he'd left his wallet behind.

Police arrived a short time later and arrested Lamb, who still had the knife in the waistband of his pants.

He was taken to Lake Illawarra Police Station and charged with robbery while armed with an offensive weapon.

Lamb pleaded guilty to the charge in Wollongong Local Court on Wednesday.

He will reappear in court on November 20 when a sentencing date will be fixed.