You should keep your PIN a secret. You should avoid using a PIN that contains repeated characters (like 1111) or digits in sequence (like 1234), and you should not select numbers that are based on discoverable information e.g. a birthday, licence number, postcode or telephone number.
You can choose to have a PIN with more than 4 digits; however some countries and companies only accept 4 digit PIN numbers. If you have a longer PIN, some of your transactions may not be successful.
No. You do not need to know your existing PIN to select a new one.
No. Changing your card PIN online is free of charge.
You will not be able to receive a One Time Password when using an international mobile number. You may be able to receive SMS to an Australian number while overseas; check with your mobile phone provider to confirm availability and cost.