A man says he was kidnapped, tied up with shoelaces and stuffed into the boot of a car before being stabbed at a busy service station in Sydney's south on Wednesday afternoon.

The man, believed to be in his 20s, will undergo surgery after he raised the alarm at an apartment block on South Dowling Street in Waterloo at 2.35pm.

Police guard the lobby of the Vista Grande apartments at Waterloo, where the man was treated by paramedics. Credit:Jessica Hromas

Paramedics treated the man for "multiple stab wounds" to his arms in the lobby of the Vista Grande apartments.

It's understood the man told police he was kidnapped, driven to Thornleigh then back to Waterloo, then freed at the Shell service station on South Dowling Street and stabbed.