under the telecommunication's act nobody, not even a cop can tap a phone without special warrants (even if the person recording is involved in the conversation).
However, if you are involved in the conversation, and all parties are present, and you feel you are threatened or in fear for your life, you will not get into trouble by using a recording device, with or without permission of all concerned.
This info may be useful for the cop's superiors if a threat did in fact take place but admissibility in court is a whole different ball game.