Nick Boudo was finishing work when he saw two men pursuing and then detaining a man who was covered in blood.
“The first reaction was ‘get off him’ because we thought the guys were attacking him,” he told Nine’s Today programme on Thursday.
“They said, ‘Nah, he just stabbed someone’.
“I said ‘he just stabbed what’.”
Billy, one of the men chasing the alleged stabber, told Nine his first instinct was “I have to stop this guy”.
The woman was taken to hospital in a serious but stable condition.
The man was also taken to hospital, under police guard, before being taken to Maroubra police station and charged.
He will face Waverley Local Court on Thursday charged with domestic violence wounding with intent to cause grievous bodily harm.