The housing and rental market has cooled down from its peak around 4-5 years ago.
While it is still competitive to get into desirable suburbs, the price has gone down on most rental listings and the line up of candidates has reduced.
Yes, there will still be competition, but it will not be as intense as it once was - expect to have some rejection but it will not be a never-ending pile of rejections.
I would not recommend organising an apartment before you move here unless you have someone your really, truly trust to inspect the place for you.
Although scammers are relatively rare, the best way to ensure you actually get what you pay for is to see the property and meet the people responsible for signing you to the lease (should you be the successful applicant).
Under no circumstances should you be paying money (either security deposit or bond) through an unofficial channel such as a private Gumtree listing prior to moving. Once you are in Perth, you can use Gumtree at your discretion but I have not used it for renting so I cannot personally recommend that approach.
Here is some additional information regarding Bond (also known as a security deposit):