The first incident occurred just after 11am when a man entered Kustom Cupcakes on Orrong Road.
He produced a silver knife and threatened the 17-year-old female shop assistant, demanding money.
The shop assistant alerted other employees, who chased him out of the shop and across Orrong Road, towards Rivervale.
He left the store empty-handed.
About five minutes later, the same man approached a 70-year-old woman who was getting out of her car on Gladstone Road, Rivervale.
He requested she drive him to the shops and when she declined he produced a knife and demanded money.
She screamed and neighbours chased him away.
He is described as 40-50 years, fair-skinned, with a slim build, blond/grey facial stubble length and rotten, worn down jagged bottom teeth.
He may be missing a few top teeth.
He was wearing a grey hooded jumper with the hood pulled over his head, dark blue tracksuit pants and black sunglasses.
Anyone with information should call Crime Stoppers on 1800 333 000.