It was reported that at around 2.39pm on Friday 2 April, a woman left the store with a bag full of goods which she did not pay for.
A member of staff, who went outside the store to challenge the woman, was then assaulted.
The victim was pushed over and threatened with a needle during this incident.
The woman was described as white and slim, with long black straight hair and wearing a black hooded top, leggings and boots. She was carrying an Asda bag for life and was accompanied by a man on a bicycle.
Officers are conducting a number of enquiries in relation to this incident, but are keen to identify any potential witnesses so we can speak to them about what they saw.
We know that a number of customers used the store and the car park around this time, so would encourage people to contact the police if they saw this incident unfold.
Anyone with information is asked to contact police on 101, quoting 44210124109.
Alternatively, you can contact the independent charity Crimestoppers 100% anonymously on 0800 555 111, or via their anonymous online form at crimestoppers-uk.org