City Fitness St Lukes gym member Matthew Le described the scene this morning as chaotic and shocking.
A man entered the gym screaming at the staff at 8.30am and was asked to leave.
When he returned half an hour later he was armed with a knife and aimed the weapon at people.
Le said he was alerted when he heard someone shouting by the door and noticed people were moving in that direction.
"I thought either they were just nosy or someone told them to get out of the gym."
Le followed the herd, as the group got to the door, the receptionist stopped them.
"The receptionist told us some crazy guy came into the gym and started screaming at the staff but that he had left now, so someone just probably had a bad case of Monday.
"So we all got back to training."
Le said when he finished half an hour later and got into his car to leave, the situation changed.
"I heard everybody in the gym running at full speed outside as if somebody was chasing them.
"I rolled down my window and asked them what had happened and they told me the same guy came back and this time with the knife chasing everybody around.
"Everybody got in their cars and left. Police arrived a couple of minutes later."
Another gym member was training in the machine section, when he saw the man wearing a black hoodie run over the entrance of the fitness centre and jump the gate.
"I heard him enter the first time around 8.10 am, there was a lot of yelling but then he left shortly after."
Police were called and everyone went back to normal, he said.
"But then again at 8.30 am, I saw the man return, at that time, he got locked inside so he ran out the back exit up the driveway.
"I saw the knife in his hand when he was running towards the machines. I started running and told people to get out the back gate."
The gym member said two police cars came to the scene and officers left with the man quickly.
"The other officers were at the scene talking to the people who hit him with a car."
A police spokesperson said they had taken a man into custody following a disorder incident at the St Lukes shopping centre.
"Initial reports of the man being verbally abusive towards the public were received after 8.30am.
