The Catholic Church has neither the motivation nor the ability to investigate complaints of sex abuse by clergy, and "the number-one issue" is to stop it doing so, a police witness told state inquiry today.

"Where is the motivation for an organisation or person to pursue perpetrators when the consequences to that organisation are so severe?" said Patrick Tidmarsh, a Victoria Police adviser on interviewing sex-abuse victims.

He told the state inquiry on how the churches handled sex abuse that the Church's handling was so inadequate and perpetrators so persistent that "I can't think of a single case where a priest has not been moved and reoffended, and moved again and reoffended again".