There is provided an anti-siphon device for securing in a fluid tank inlet. The anti-siphon device comprises a housing, an inlet to the housing adapted to receive a fluid dispensing nozzle, the inlet at a proximal end of the hosing, and an outlet at a distal end of the housing. The anti-siphon device further comprises an obstruction disposed within the housing, where an outer surface of the obstruction and an inner wall of the housing define a first flow passageway. The obstruction comprises, a chamber disposed within the obstruction and configured to receive fluid, an inlet aperture in a first portion of the obstruction, and an outlet aperture in a second portion of the obstruction, where the inlet aperture, chamber, and outlet aperture define a second flow passageway, the second flow passageway being configured such that, in use, fluid can flow from the inlet aperture to the outlet aperture via the second flow passageway, whilst flows from the inlet to the outlet via the first flow passageway.