The 39-year-old responded to the opposition National party's new law and order policy on Sunday, but hasn't been seen at parliament since.
She revealed in an Instagram post - alongside a picture of herself holding a small puppy - she needed to step away.
"Over the past few weeks I've really been struggling with mental health and wellness. Triggered by personal circumstances, as well as other external things - even things like the cyclones having such a massive impact on our entire region," Ms Allan wrote.
"Sometimes things accumulate, and I hit a wall a few weeks ago. I kept going, showing up to events, smiling, giving speeches.
"About a week ago, I had to be pretty honest with myself that I needed to put my mental well-being at the forefront of my focus."
Ms Allan said it was "bloody hard" and "a little embarrassing" to tell those close to her she had been suffering.
"What followed was an overwhelming response of aroha - and I am so glad I shared with people I love to help pull me out of my head," she said.
"So if you've been wondering why I haven't been at work this week, it's because I had to take some time to get some help and get back on track.
"I'll be back at work tomorrow with a little skip in my beat - but honestly, it's taken a little while to get there."
Ms Allan said she would be back at parliament on Thursday.
The Labour MP's announcement came before a story broke of cultural concerns in her office.
News outlet Stuff revealed that a seconded public servant asked to leave the office due to relationships, triggering an internal investigation.
Ms Allan, a former intern in Helen Clark's prime ministerial office, was first elected in 2017 and promoted into cabinet in 2020.
In April 2021, she took leave after being diagnosed with cervical cancer and told she had a 13 per cent chance of survival.
She returned to cabinet with a lighter ministerial load in July.
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