The Law Society has told MPs proposed changes to the child support system are complicated and do not provide enough certainty for single parents.
Parliament's Social Services Select Committee is hearing submissions on the Child Support Amendment Bill.
The legislation aims to change how child support is calculated, by taking both parents' income into account rather than just the paying parent.
The number of nights a year used to determine shared care will also be reduced from 40% to 28%.
Law Society family section deputy chair Catriona Doyle told the committee parents will need a lot more help and support to work out the new calculations.
Ms Doyle said the reduction in the number of nights a year used to determine shared care is not fair to both parents.
She says shared care is not necessarily a true reflection of the shared costs of raising a child.
Ms Doyle says women and children are more likely to be affected by the changes.