Hopely, comes to Perth sometimes either July or August or in between this year.
I need to by smart card for public transports for a few or so days to stay in Perth. (Which fares I need for a week).
It depends on how long you intend to travel around Perth really. If you will be here during the school holidays - 7th July to 23rd of July - and traveling with a companion, the best option is to ignore the smart card (SmartRider) and purchase a "FamilyRider" paper ticket each day you travel, so long as you start each day after 9 in the morning. (The FamilyRider is valid on all Transperth-branded services, all day on weekends and public holidays, after 9 am on school holidays, after 3 pm on school Fridays and after 6 pm school Mondays to Thursdays.) At present, this costs $9.30, although it is just about a given this will rise by around 3% at the beginning of July.
In other situations, it gets more complicated. If you will be traveling a lot - meaning frequenting the extremities of the network for extended periods of time - then a "DayRider" cash ticket is available - it is the same as the FamilyRider, except valid after 9 am every weekday (including school days) but only for one person. If travel is less intense, the SmartRider smartcard would be appropriate as this applies a 15% discount to regular cash fares, plus caps fares for each day at the DayRider price. It should be noted however that the adult/standard SmartRider does not discount the DayRider cap, however it is 'harder' to reach so will often cost less than the cash ticket. SmartRider is available at Transperth InfoCentres at Perth and Perth Underground train stations, plus Wellington Street and Esplanade bus stations, as well as at authorised Retail Sales Outlets listed at http://www.transperth.wa.gov.au/TicketsandFares/SmartRider/BuyingSmartRider/RetailSalesOutlets.aspx
. The SmartRider costs $10 to purchase ($5 for a concession card, although you need to register for these), plus money needs to be added to the card - see http://www.transperth.wa.gov.au/TicketsandFares/SmartRider/AddValueMethods.aspx
for details on this. Once purchased, the SmartRider works exactly like myki money in Melbourne - tag on and off all buses, ferries and trains (special rules apply at Whitfords and Warwick train stations, where tagging on a bus will automatically tag off the train, and vice versa.) Unfortunately, the $10 fee is non-refundable, as far as I'm aware.
Unless you will only be in Perth for a couple of days, I would steer clear of the cash 'zone' fares and 2 sections fare - even with the card fee, SmartRider will generally come out in front.
More information - as if this waffle wasn't enough - can be found at http://www.transperth.wa.gov.au/TicketsandFares.aspx
PS. None of the above applies to the Rottnest ferry, which is not a Transperth service and has its own fare structure, or to any regional train services i.e. Australind and Prospector. SmartRider can be used on some regional buses though.