A man accused of a series of murders during the 1980s over a long-running custody battle with his estranged wife had benefited from "very good" Family Court orders for a man at the time, his lawyer has told a court.

Instead police allegedly failed to turn their mind to Leonard John Warwick's success as a litigant when they pursued him over the deaths of four people, including a judge who presided over his case.

Accused Family Court bomber Leonard Warwick awaits the verdict of his long-running criminal trial. Credit:AAP

Solicitor Alan Conolly told NSW Supreme Court Judge Peter Garling his client had suffered from a "completely misdirected police investigation" despite being granted sole custody of his daughter in 1986 and a deed of settlement regarding a house he shared with his former wife, Andrea Blanchard, the following year.

"Your Honour, all this terrible, terrible trial is about is a very basic detective failing, and that was to understand the Family Court file," Mr Conolly said during the defence's closing address in the 23-month criminal trial.