do i need to use seatbelt with isofix

Short answer: with an ISOFIX child seat, you use the ISOFIX anchors to install the seat—not the vehicle seatbelt.

In Australia, you must also attach the seat’s top tether to the anchor point, because our standards require it for most restraints.

For ISOFIX-compatible boosters, the ISOFIX holds the booster in place, but the child is restrained by the car’s lap–sash seatbelt.

Never use ISOFIX and the seatbelt together unless the manufacturer explicitly says so; usually it’s one or the other.

Read the restraint manual and follow your state’s child-seat laws—then bask in the smug glow of a properly fitted setup.