The head of one of Sydney's top private schools has warned that denying children access to their fathers when the parents' relationships break down can lead to girls seeking "inappropriate contact" with other males, and boys rejecting their mothers.

In a newsletter to parents, John Collier from St Andrews Cathedral School said he had seen the ramifications of separation for almost 30 years, watching parents become so absorbed with their own battles that they overlooked the impact on their child.

Dr John Collier, the principal of St Andrews College, has urged warring parents to stop for the good of their child. Credit:Louie Douvis

"Presumably subconsciously, [some parents are] prepared to forfeit the interests of the child for the sake of tactical advantage in the ongoing battle with their ex-partner," said Dr Collier. "When parents are at war with one another, my experience is that the child almost invariably suffers."

Elisabeth Shaw, NSW chief executive of Relationships Australia, said separation was an "intense, volatile" time, and parents sometimes became so caught up in their pain that they could not see the ramifications for their children.