Police said the 47-year-old man was allegedly caught stealing from the BJ Howes industrial equipment store at Lisarow on Saturday when a fight broke out with a staff member. It is believed the store worker chased the suspected thief and tried to stop him leaving.
The man then suffered a suspected heart attack outside the front of the shop.
When police officers arrived they gave the man CPR until paramedics arrived.
He was taken to Gosford Hospital and remains in a critical condition.
A police spokesman told the ABC that it is sometimes best to leave arrests to the police.
“Our main concern is safety of all members of the public and we don’t necessarily encourage people to do what’s known as a citizen’s arrest,” Inspector Nigel Webber said.
“We encourage people to contact us straight away and let us deal with it.”
Police have appealed for witnesses to come forward.