He later revealed he was struggling with anxiety and panic attacks, something that he had never experienced.
Muller's wife Michelle was his support person during this time, and he saw the strain it put her under, he told The Country's Jamie Mackay.
"I know from experience that it was a huge stress on Michelle last year when she was putting the emotional scaffolding around me to try and stop me falling."
Listen to the full interview below:
"I think that happens in a lot of homes in New Zealand."
Muller believed it occurred in rural New Zealand homes as well, with those supporting a farmer in need.
"They can see the farmer's out isolated, falling, and can see the pain and anguish that he's going through."
"But the acute stress and anguish for the partner or wife to try to assist him through that – that is often not talked about."
There needed to be support for family members and partners who were "often deeply anguished themselves, although not going through a mental health episode," Muller said.
"That's got to be part of our conversation."
It wasn't only Michelle who helped Muller through his experience, as he also drew inspiration from Sir John Kirwan's mental health message.
"I don't think I could still be in this job if it wasn't for John Kirwan all those years ago normalising the fact that these things happen for a lot of New Zealanders," he said.
"A politician falling over like I did last year? That would've been the end of it 10 years ago."
Muller's mental health was "much better" these days and he credited Kirwan's influence.
"Huge thanks to him, he does a remarkable job."
Also in today's interview: Muller talked about how globalisation was affecting New Zealand agriculture and indulged Mackay in his fascination with the Ever Given in the Suez Canal.
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.