The Cabinet Minister and MP for the Northern Tablelands revealed he didn't take a single sick day during the nearly three weeks he spent in isolation with the deadly pandemic virus, despite spending days in bed.

"I never thought getting up and going for a run would feel so good, but it did this morning. It's the first time I've been able to go for a run for 20 days so that was very, very nice this morning," he said.

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It could have been far worse. He was just hours away from causing a potentially "catastrophic" outbreak in NSW parliament after contracting the virus in June, Mr Marshall said.

He praised NSW Health contract tracers for identifying him as a potential carrier before he could infect the Premier, other MPs, or parliamentary staff.

"[When I was informed I could be infectious] I just felt physically ill, not because of COVID, I just felt sick about the fact that I had it and the implications that had for all the people, hundreds of people that I'd been in contact with for the previous 24 hours after I had been exposed," he said.

"Thankfully though the system worked exactly the way it should, the QR code system, the contact tracing. And I was able to be notified and go into isolation; essentially before I became infectious.

"If it had been just 12 hours later the impacts could have been catastrophic."

Mr Marshall said he was "grateful" that his infection had been detected in Sydney and avoided what he called the worst-case scenario of bringing it back to rural NSW.

"I'm just incredibly grateful and, in a weird way, am counting my lucky stars. I don't think I would have been able to forgive myself if that scenario had played out," he said.

The MP spent his entire nearly three weeks in quarantine in a Sydney hotel.

He was released at about 7am on Friday after being ruled no longer infectious by health authorities.

Mr Marshall, who is in his mid-30s, told the Leader the Delta strain had "knocked me flat in a way I was not prepared for, way worse than a bad flu. It is not to be taken lightly".

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"I kept working throughout. It was quite difficult for four or five days where I was sort of bed-ridden and really even struggled to prop myself up in bed, but I had my phone with me, my ipad, laptop, so it was all sort of doable," he said.

"If you're fortunate enough to be eligible for a vaccine, bloody well go and get a jab in the arm. I wish I was eligible, because I would want to get vaccinated as soon as I could.

"Vaccination equals freedom."

Mr Marshall drove back from Sydney on Friday.

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