Unless you have a formal agreement or order, under the Family Law Act, parents are each a guardian of the child where they have lived with that child.
An informal arrangement does not impact the legal guardianship status of a parent. Only a court order or an agreement can remove guardianship.
Both parents will be legal guardians in the scenario where, since separation, one parent has had all of the care and responsibility of the child, and the other parent has been uninvolved. The court, in this situation, could limit that parent’s parental responsibilities, or even remove guardianship from that parent if appropriate.
Where there are informal parenting arrangements in place, a parent cannot decide on their own to change these arrangements. For example, if one parent has not been involved, they cannot decide to pick the child up from school for a visit without consulting with the other parent.
Similarly, if both parents are involved and there is a routine, for example one parent picks the child up from school Tuesdays and Thursdays, those arrangements cannot be changed by one parent deciding on their own.
If you have informal arrangements, either parent may seek an agreement or order to formalize parenting arrangements and resolve any disputes.