If you find a litter of kittens, please leave them where they are. Their mother might have heard you coming, so you can't see her, but she's probably hiding somewhere nearby. Kittens have the best chance of survival if they are with their mother, especially if they are very young.
If you are sure their mother is not around, and the kittens are too young to survive on their own (e.g. their eyes are still closed and their ears are pinned down), or if they appear in need of urgent medical treatment, call the RSPCA Welfare Hotline on 1300 278 358. If you are nearby, you can bring them to the RSPCA Animal Care Centre in Malaga.
If the kittens are a little older and appear healthy, please contact your local city or shire rangers. Your local government is responsible for lost or stray cats under the Cat Act 2011.
You could also contact one of the many cat rescue groups who will be able to find loving permanent homes for them:
Cat Haven08 9442 3600 9 Lives Cat Rescue0408 567 204 Perth Animal Rescue0416 020 974 Impound Feline Rescue0497 898 769 Paws Cat Rescue0414 875 235 Small Paws Rescue0403 367 265 Mad Catters Rescuemadcattersrescue@outlook.com.au  Perth Rescue Angelsmeow@perthrescueangels.org