Tank inlet seal
A leak under the tank
Breville's US manual: "For any maintenance other than cleaning, call Breville Consumer Support." Sage's UK guide: "Any maintenance other than cleaning should be performed by an authorised Sage service centre." Opening the case is an unauthorised repair under Breville's and Sage's warranty terms and voids the warranty (1 year in the US and Canada, 2 years in the UK, EU and Australia). Wait until it has run out. Treat this as service-centre work: the methods shown are owners' and retailers', named where they appear.
First, the owner's checks from the manual: the tank must be pushed fully down, and the drip tray in and emptied.
If water still collects under the tank, the tank's inlet seal (Breville "Water Inlet Seal", SP0001510) is the part to suspect. Two screws, reached from below, hold the black inlet piece on the back (Aju's Cafe, Seth Morris, owners). City Master Appliance (owner, 2025) warns that leaving the inlet part out on reassembly makes the tank leak badly.