Todd Muller is no stranger to stressful situations, but his reaction to his brief stint as Leader of the Opposition caught him by surprise.

Muller was National's leader for just 53 days before announcing he was stepping down from the role for health reasons.

He later revealed he was struggling with anxiety and panic attacks, something that had never experienced, despite other high profile roles that were "right up there in terms of stress," he told The Country's Jamie Mackay.

"I'd had reasonably big job at Zespri and certainly bigger at Fonterra, when I was there we had botulism and DCD and huge stress, you know?"

While Muller had a "bit of a challenge sleeping" during this time, he said he "got through it well."

However, the stress of being National's Leader was a very different experience for Muller.

"But this was completely something else again. Just entirely debilitating and because it caught me by surprise ... it became a very, very dark journey for me."

Muller said he began to develop physical symptoms, such as a "serious prickling" in his head. This was followed by "waves of anxiety that just got worse and worse."

These physical reactions were coupled with pressure he put on himself about what others would think, along with the "huge fear of failure" and what that could mean for his reputation.

"There you are, you're the Leader of the Opposition, you've finally got to a position that clearly you wanted and the caucus felt you would be good at - and then you start experiencing this."

"I was worried about what my wife would think, what my kids would think, what my wider family, community, clearly the party and caucus [would think]."

"So you just wrestle with yourself - I did - behind closed doors."

Muller also had trouble sleeping and would dry retch, which he thought was a way of trying to "vomit up the anguish."

"It's quite debilitating and I'm sure there are people listening who have been through that sort of experience and they know from first hand - when you're there - it's really, really tough."

Muller said he "absolutely loved" the first few days as National Leader, but his anxiety manifested around day five of his new role.

"I was driving back from Auckland in the car and then suddenly, out of nowhere, I had this intense physiological reaction - it felt like my head was in a vice and then those waves that I'd talked about - and I'm sitting in the back seat doing deep breathing, hoping that the driver doesn't see me."

"When I got home I really just burst in to tears - which is not something I do to be honest - I did it when dad died a few years back - but I tend to be someone who compartmentalises all these things, because there's just so much going on."

Muller said he "sobbed deeply" in front of his wife "for a long, long time", which helped alleviate some of his anxiety for a few days but then "bang - it hit again."

The experience was "intensely draining" and Muller said he could now see the effects in photos of himself at the time.

"You can see it in my face, at about five or six weeks ... I just looked different. I looked grey and thin - it was defeating me."

Compartmentalising and "buttoning up" emotions, while telling himself to "grip it up" didn't help Muller deal with tough life-situations, such as the death of his father.

"I think what happened is that I rather too quickly moved on - or thought I was moving on - to the next stages of both professional and personal life, without actually reflecting."

Muller said his mental health experience gave him insight into how farmers might feel overwhelmed at times.

"I'm passionate about rural New Zealand. I'm from that background. And it's made me, I think, in a small way, more empathetic of what it must be like when you're out the back of the farm and it's just feeling bloody tough and you just don't want to talk to anyone about it so you just hold it inside."

"You're almost too fearful to tell anyone because it might be too overwhelming to let it go - the sense of failure - all that stuff that I had. I was terrified my reputation was just going to be smashed."

Although Muller said was feeling much better, getting back on track wasn't something that happened overnight.

"I'm not perfect. You don't flick a switch with these things, but I'm certainly a whole lot better than I was."

He said he still had to deal with his short tenure as National's Leader.

"It sits with me. I can't relive the fact that I was only the leader for 53 days - that's what it is - but my goodness I feel a whole lot better being able to share it and talk about it and quietly rebuild the other side of it."

Muller said he is a different man as a result of his experience, and now wasn't as interested in the political issues he would have obsessed over in the past.

"I've changed a bit to be fair. The Todd Muller of a year ago would've poured over the announcement of who sits where and who's what number, and who's got what."

"That stuff's so far away from my mind."

Muller said he was looking forward to his new role as Nationals' spokesman for Trade, rather than worrying where he stood in the party.

"I love trade - I don't give a stuff what number I am."

Also in today's interview: Muller spoke about the RCEP deal and commented on Simon Bridges' National leadership during Covid-19.

Where to get help:

Rural Support Trust: 0800 787 254
Lifeline: 0800 543 354 (available 24/7)
Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
Youthline: 0800 376 633
Kidsline: 0800 543 754 (available 24/7)
Whatsup: 0800 942 8787 (1pm to 11pm)
Depression helpline: 0800 111 757 (available 24/7)

If it is an emergency and you feel like you or someone else is at risk, call 111.