訂機票問題

You can try Flight centre
http://www.flightcentre.com.au

There are high and low seasons. Depending on when you plan to travel.
For Direct Flight to HK from Perth, you can travel on Qantas or CX.

Check around with other travel agents and if you give them your cheapest quotation, Flight Centre will be able to give you a better offer or match the price quoted.

20 kg are the usual weight allowed for check in luggages but usually they will let you through for 25kg. Subject to the airport check-in staff of course.