There are warnings that a three-year investigation of a bugging scandal that rocked the upper echelons of the NSW Police could be further delayed amid questions about the ombudsman's ability to make adverse findings.

Acting NSW Ombudsman John McMillan has previously revealed that provisional findings have been made against 33 people in the investigation, Operation Prospect.

Acting ombudsman John McMillan.

But in a statement published on Friday in response to a request for an update on Prospect's progress by a parliamentary committee, Professor McMillan says he expects it can be finalised in the first half of 2016.

But he also raises questions that Greens MP David Shoebridge - a committee member - believes could leave Professor McMillan facing "a raft of legal challenges".