Books have been dedicated to addressing selection criteria and there is a ton of information freely available on the Internet.
I am not going to try and re-invent the wheel as far as these are concerned. Generally speaking, if you're applying for jobs in the public sector then you will have to devote a great deal of time and effort to complying with the requirements of the application process.
Why are they used? Organisations in the public sector have to be very objective in the selection process, with a complete lack of bias. Careful reading of the evidence provided by the candidates results in a more efficient sifting process for the first interview. At the interview, the panellists will delve into your answers to the criteria in more detail analysing the truthfulness of your statements. They will then use a matrix-based assessment process to determine the best candidate.
I do not write these for individuals. It would simply take too long and would cost you a lot of money. After all, you're the one with the experiences and the evidence you need to satisfy the requirements.
However, I am very happy to review and edit the way you have addressed the criteria for any job, so contact me if you need any help.
To get you started, here's a useful link. You'll find that the STAR model is the one most commonly used.
Click here for a very informative guide issued by the Australian Government.
If you enter 'STAR model + selection criteria' into Google you will find plenty of information on this topic along with links to publications you can purchase.
One book that I have found very useful is How to Write and Talk to Selection Criteria by Dr. Ann Villiers. If you're interested in learning more or purchasing a copy then the link will take you to the relevant page on Dr. Villiers' website.