If there is one tip visitors should take, it is this - get on public transport after 9am (on weekdays) and buy a DayRider. If there are two of you, it will also only cost $10 for both of you - buy a FamilyRider which only costs $10 for two standard fare passengers (and includes up to 5 more children if they have an Australian concession card). This entitles you to ride the whole public transport system all day (from 9am if on a weekday). Does the FamilyRider or DayRider include the Airport? Yes it does! Here are the usual fares for Perth as of March 2023.
Also, if you are using the Smartrider, note that you will never be charged more than $10 per day if you use public transport multiple times.