Police were called to the Eglinton Rd, Dunedin, shop just after 10pm, after a report a knife-wielding man made off with cash and tobacco products from the store.
He was last seen running across Mornington Park, and his track was later followed for a short distance by a police dog.
Detective Constable Simon Forde said that a search of the park yesterday resulted in the recovery of a knife.
"This knife is obviously of interest to our inquiry."
The male shop assistant was left shaken but unhurt by the incident.
The assailant was described as Caucasian, aged between 20 and 30, of medium build and about 170cm tall.
He was dressed in black clothes and had his hoodie pulled over his head. A black scarf was used to conceal his face, and he wore what appeared to be yellow leather gardening gloves.
Dairy owner Qian Sun said the robbery was the fourth in a year and was "not good".
She said that another robbery was "frustrating", but she remained committed to the business.
Det Const Forde said anyone who knew of the offender, or recognised the knife, should contact police on (03) 471-4800 or Crimestoppers on 0800 555-111.