"Don't know who the young guy was but good on you and thank you," she wrote.
A police spokesman this evening wouldn't confirm the witness account, saying no further information would be released apart from the fact the armed offenders squad was not involved.
Police say a small amount of cash was taken from "a commercial premises on Beach Rd" about 2.30pm today.
Nobody was injured and officers are looking for the robber.
The Mobil is open as normal this evening and there is no police presence.
Anyone with information can contact the North Shore CIB on 09 477 5261.