As Taka said, ice skates are specifically prohibited as carry-on under Australian regulations, so have to be packed in your checked luggage.
You do not need to declare them at check-in, or to Customs on arrival in Sydney (but, off-topic, please do take the questions on the Customs Declaration form seriously and answer honestly if you wish to avoid a fine).
If you arrive at the airport in plenty of time for check-in, have allowed plenty of time between any connecting flights, and your bags are labelled with your name, it is very unlikely your luggage will be lost. In many years of travelling, I have had my baggage delayed a few times, but on those occasions it has always reached me safely within 12 hours of arrival. My husband used to travel regularly for work, making several overseas trips a year (between Australia/Europe, Britain/Europe, Britain/US, Australia/US) and has never had a bag lost. This includes lots of trips carrying skates in checked bags.
Also, please excuse my pedantry, but its Qantas, not Quantas. Qantas was originally an acronym for Queensland And Northern Territory Aerial Service, hence does not have a 'u' after the 'q'.
EDITED TO ADD:Husband said under no circumstances check your skates at the gate, because if you check them at the gate the bag then gets thrown down a chute two stories to the ground. Which would be a great way to get your blades damaged. Much better to pack them well, padded with clothes, in a hard suitcase with a proper tracking label applied at check-in.