It is a car seat here's the video from Safety First on Youtube:
Safety 1st Custodian Plus II Convertible Booster Seat, Baby Mode Australia
The Custodian Plus Series II Convertible Booster Seat grows with your child.
Suitable From: 6 months to 8 years
This seat has two travel modes. As you child grows you can easily convert this seat from built-in harness mode to Adult lap-sash mode which will last your child to approx 6-8 years of age.
To be used in forward-facing position with built-in harness from approx. 6 months to approx. 4 years or until the child's shoulders reach the middle shoulder height label.
To be used with adult lap-sash seatbelt from approx. 4 years to approx. 8 years or until child's shoulders reach the upper shoulder height label.
Also the narrow seat designs allows for greater passenger space in your vehicle. You can fit three Custodian Plus ll Booster in most family cars
Featuring the superior side impact protection system - Air Protect and enhanced features including an extra 3cm in the crotch area for added comfort, 2 piece harness system for easier use and limited twisting, cup holders to store drinks and snacks and silicon arm rests to protect their arms from the heat.
Dimensions Assembled:45W x 43D x 77-86H cm