I want to take a cardboard box size 22x18x18 weight 35Lb to Sydney Australia on Qantas airline as a checked bag. Do you think there will be any issue to take it?
No, this will not be a problem - and I can say that as someone that has done it several times. The box will count as one item of your checked baggage - no different to what a suitcase would.
At check-in it's likely you will be required to sign a waiver that the airline will take limited responsibility for the contents - this is fairly common when using a box.
It's likely your box will not be delivered to the standard baggage claim carousel, but instead to the oversized item area, although on occasion I've had them put it on the normal carousel so keep an eye on both. If you're arriving into Sydney on an international flight, the oversized area is off to one side of the baggage hall. If it's domestic, it'll be in the oversized area between baggage carousels 3 and 4.