Malaysian Police are able to arrest people wanted in Singapore because particulars of "Wanted" criminals are shared with them and vice versa. Do remember we were once part of Malaysia and our SPF was once Polis Di-Raja Malaysia. Our Police officers also trained with Malaysians at the Police Training Depot at Kuala Kubu. These Malaysian trained police officers have all since retired.
But the processes and systems remains in place. "Wanted" persons are sent across the Causeway without fuss or need for international extradition court orders - it's still an abang/adek brothers-in-arms relationship.
Singapore criminals are silly to believe that they can hide in Malaysia...... In spite of short-comings, the Malaysian Polis is a professional body. Their Special Branch is to be feared and respected. Even LKY got a shock during the merger times (1963-1965) when Special Branch revealed to him that one Singapore MP was their informer.