The incident, which involved a man and a woman, took place on Thursday between 14:20 and 15:00 on La Grande Route de St Pierre opposite the Sir George De Carteret pub.
A man has since been taken into custody, and will remain there while Police enquiries continue.
The man involved is described as being in his 60s, with grey hair and of a slim build. He is reported to be around 5ft6, and was wearing a black coat and bright multi-coloured trousers.
Pictured: The bus stop where the alleged sexual assault took place. (Google Maps)
The woman was wearing a purple raincoat and black trousers.
Police are now appealing to anyone who may have been in the area or have dash cam footage from the time of the alleged assault.
Those with any information are asked to contact Police on 612612 or anonymously via Crimestoppers on 0800 555 111.