Around 22.35pm two masked men threatened the two women at the Old Toll Garage in Main Street with knives and demanded money.
Both suspects are described as white, around 5’7” tall, of slim build, and wearing dark clothing and balaclavas or similar covering their face.
They made off on foot to the rear of the petrol station with a three figure sum of money and a quantity of cigarettes.
A 36-year-old woman suffered a cut to her shoulder as she tried to stop the robbery, while an 18-year-old woman was restrained by the suspects, but was otherwise injured.
Both were treated by paramedics at the scene, but did not require hospital treatment.
Detective Inspector Kenny Dalrymple, Wishaw CID, is leading the enquiry.
He said: “This attack was obviously planned. The suspects waited until nearly closing time, possibly hoping to get the days’ takings and maximise their haul.
“However, they were thwarted by the staff in what was a very, very frightening experience for them both.
“Officers are currently checking CCTV from the scene and have spoken to people who were on the forecourt at the time of the attack.
“We are keen to hear from anyone who was in the area at the time, particularly any passing motorist who may have dash-cam footage.
“Main St is one of the key routes into Wishaw and would have been busy around this time of night, so if you saw anything, or have any information that will assist our enquiry, then please contact Wishaw CID via 101 or call Crimestoppers in confidence on 0800 555 111.”