Bill English has blamed National's failure to respond to an invitation to meet families of suicide victims on an "administrative" error.
The families wrote a letter at the start August, asking representatives from all the parties to meet with them on September 11 to talk about suicide prevention.
The Greens, Labour, New Zealand First, United Future and the Māori Party all responded. National and ACT didn't.
"It was good enough for all the other parties to acknowledge and agree to attend. I don't really understand why our current Government can't see fit to actually speak with us," said spokeswoman Jane Stevens.
"I would urge them to come and actually face up to the families and listen to what they have to say."
Mr English told The AM Show on Monday he wasn't aware of the invitation, but someone from National would definitely meet them.
"I'm sure we'll have someone there. We're all concerned about suicide - I know about it, I've seen its devastating impact."
As for why the party didn't even RSVP, he wasn't sure, saying he's been too focused on the election campaign.
"Look, there will be some administrative explanation for that... I'm sure there will be someone down to go, I've just got to go and find out. I don't know the detail of that, but I'll be happy to find out."
On Sunday, more than 600 shoes were placed on Parliament's front lawn, representing the number of Kiwis who have committed suicide in the past year.
Labour leader Jacinda Ardern spoke at the event, choking up as she spoke about the suicide of her best friend's brother when she was 13.
If you wish to talk to someone about mental illness or domestic violence, you can call Lifeline on 0800 543 354, the Depression Helpline on 0800 111 757 or the National Telehealth Service on 1737.