You went for a walk at Cardinia Reservoir or a swim down at Lang Lang, came back, and the keys are not in your pocket. It happens regularly enough that we have a routine for it.
Step 1: do a proper search
Before driving home or making any calls, retrace your steps carefully. Check picnic blankets, beach bags, towels, glove boxes, under car seats, between the seats. People often find keys after the initial panic by stopping and thinking through the last 30 minutes.
Step 2: if they are well and truly gone
For car keys, the situation depends on what type you have. A modern smart proximity key is hard to replace without the dealer because of the immobiliser. An older mechanical key is easier and cheaper for a mobile locksmith.
For house keys, if you have a spare with anyone, this is the moment to use it. If not, you need either a locksmith to open the door (the spare is inside) or to access the house another way.
Step 3: getting home from the park
If you are stranded at a park or beach with no car key, options are: call a partner or family member with a spare to bring it to you. Call a mobile auto locksmith to come to your location. In rare cases (modern smart keys you only had one of), you might need to call a tow truck and have the car taken to the dealer.
What you will need for the locksmith
Photo ID showing you match the car registration. The car needs to be in your name. If it is a company car or you cannot prove ownership, the locksmith cannot legally help.
Typical costs
Coming to a park or beach to open a car: $180 to $280 depending on distance from Pakenham. Cutting a new key for an older car: $80 to $200 once at the locksmith's workshop. Smart key programming: $300 to $1,200 depending on make and model.
Prevention
If you go to Cardinia or the coast regularly, consider a key tether that attaches your keys to your bag or wrist. Keep a spare key at home with a neighbour or friend. For your car, look into whether you can have a hidden spare key holder fitted to the chassis (some people do this; it is not for everyone).