The Australian Federal Police (AFP) have won a court application to keep a young man arrested in counter-terrorism raids in Sydney last week in custody for another 68 hours.

The 18-year-old was among four men arrested when state and federal police raided homes across Western Sydney on Wednesday morning.

The raids were sparked by the execution-style terrorist shooting of police accountant Curtis Cheng outside NSW police headquarters in Parramatta on October 2.

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The 18-year-old can be held in custody without charge until Thursday evening.

Three of the men arrested last week were released without charge the same day.

On Monday, Prime Minister Malcolm Turnbull cautioned those who tried to tag all Muslims with responsibility for the crimes of a tiny minority, were undermining the national interest.

The Prime Minister again offered his condolences to the family of Mr Cheng,  in a terror attack carried out by 15-year-old Farhad Jabar following Friday prayer.