David Eastman.

David Eastman. Photo: Andrew Taylor

Lawyers for David Eastman have fought back against a challenge that could stop the inquiry into his murder conviction in its tracks, sparking a fresh battle that could end up in the High Court.

An inquiry is currently looking at the conviction of Eastman for the slaying of the ACT's police chief, Colin Winchester, in 1989.

That inquiry has already heard a substantial amount of evidence and is adjourned until January.

But before the summer break, the Director of Public Prosecutions launched an application for judicial review of two crucial decisions that have helped create and shape the inquiry.

The DPP challenged the legality of a decision by Justice Shane Marshall to order the inquiry in August last year, and a decision by acting Justice Brian Martin last month not to limit its scope.

The full bench of the ACT Supreme Court began to hear the DPP's challenge on Monday.

But lawyers for Eastman hit back early, arguing there was a legal barrier that prevented the judicial review of a decision to order an inquiry. That has raised questions of constitutional law, which will be dealt with separately to the DPP's current challenge.