I’m going to travel to india from Australia. As I’m going to add 5 kg extra baggage allowance. Somebody told me if i add extra allowance then the custom at Delhi Airport gonna do checking and i may have to open my bag. Is that true?
Weight of your baggage is between you and the airline. Customs has no way of knowing what the weight of your luggage is so they can't use weight as an indicator for extra scrutiny.
Sometimes the airlines will put a sticker stating "Heavy" or similar on your bag so that the baggage handler can follow proper protocol for overweight luggage. If you are worried about that, you can simply remove any stickers after you have collected your bags from the belt and before you take them through customs