Mr Mitchell called time on his 40-year career on Thursday, with the 58-year-old marching from Newcastle Police Station down church street, saluting his friends and colleagues - with two officers from the Mounted Unit following him and the PolAir helicopter passing overhead until he reached a waiting highway patrol car, climbed in and was taken to a new stage of his life.

NSW Police Commissioner Mick Fuller told the Newcastle Herald Mr Mitchell would leave "a massive hole" and would be "absolutely missed".

"He's been an outstanding leader for not just Newcastle, but all the areas he's worked across for the NSW community," Mr Fuller said.

"But he's also a great police officer and I think what's unique is he was a working man's police officer - he came through the ranks and really in terms of what he's achieved, not only for the Greater Newcastle area but northern NSW, is outstanding."

Deputy Commissioner Gary Worboys and Police Minister David Elliott were also in the city to wish the retiring Assistant Commissioner well on his final day at work.

Mr Mitchell grew up at Lake Macquarie - the son of a Chief Inspector - and joined the force in 1981.

He spent most of his career based in and around the Hunter - first as a general duties officer in a range of commands in the region and then, after he was made a detective in 1989, at places including Raymond Terrace, Tuggerah Lakes, Brisbane Water and Newcastle City.

Mr Mitchell was instrumental in the development of the Transport Command - which covers the state's public transport - as well as the creation of the Aged Crime Prevention Officer role in the force.

In 2011, he led a team of the state's police to New Zealand's South Island in the days after the devastating and deadly Christchurch earthquake to help with the recovery mission in the city.

"When we arrived, within 72 hours, it truly was like a war zone," he told the Newcastle Herald earlier this week.

"The devastation was incredible.The level of thanks from the Christchurch people - even to this day, if they knew you were involved in the earthquake support they have nothing but high esteem and kind words to say."

Mr Mitchell finished his time based in Newcastle as the commander of the NSW Police Northern Region - which stretches from the Hawkesbury to the Queensland border.

He has received the Australian Police Medal and the National Police Service Medal - along with various clasps - as well as a Humanitarian Overseas Service award and a Commissioner's Commendation.

In reflecting on his career earlier this week, he said he did not regret a day on the job.

"There have been some ups and downs, of course, but if you turned back the clock I'd do it all again," he said.

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