Ross told Stuff he did not intend to speak at the event – "I'll just be there quietly applauding the exceptional people that work in mental health, and thank them for what they do supporting patients during some very difficult times".
"I hope to see some of the doctors and nurses that were so kind to me while I was in their care."
Health Minister David Clark is due to give a speech at Middlemore at midday.
A spokesman for the minister said he was not aware that Ross would be attending the event until it was reported in the media today.
Ross' appearance today comes after a turbulent few weeks for the MP.
In early October, National Leader Simon Bridges said Ross would be absent from Parliament for a few months while he dealt with personal health issues.
A few weeks later, Bridges said the PwC investigation into the leak of National Party travel expenses concluded that on the "balance of probabilities the evidence establishes that Jami-Lee Ross was the person who leaked the expenses and the sender of the text message".
The next day, in an hour-long media stand up, Ross launched a scathing attack on Bridges over claims of electoral fraud – claims that Bridges denied – Ross resigned as a National MP, but said he would stay on as the local MP for Botany.
He then proceeded to leak secretly record phone conversations between himself and Bridges to media.
Ross was taken into care at the mental health unit of Middlemore later that week.
He has not made a public appearance since he was discharged on October 23.
