Staff at the government agency found a mistake in how they applied the Data and Statistics Act (2022).
Over the next few hours, officials are hoping around 51,000 New Zealanders fill out the form.
Māori and Pasifika are lagging behind, with about one in four yet to return their forms.
ACT says it's "not even in the realm of sensible and targeted spending".
"I predict this move will get Census completion rates near 100 percent overnight."
She won't commit to standing down if the Census doesn't hit.
"I said, 'well, what do I do if I get another letter about it?' And he said, 'Just throw it in the bin'."
"You can submit your forms anytime in the next couple of days or even weeks."
"I'd always hope for a higher degree of response before census."