A flushing machine worth £740 was stolen from central heating engineers as they did repairs in a council house.
The Kamco tool used for removing water from pipes was pinched after workers left it on the pavement between their van and the Westfield Road home in Northchurch.
Police were called at about 4pm on Friday.
Sgt Peter Huffer said: “While this is an unusual situation, it does show that an opportunist thief can take materials within seconds.”
He reminded workmen to secure their tools at all times.
If you can help police solve this crime, phone them on 101, citing crime reference D2/12/73.