Her Covid-19 infection comes as a big week in Parliament gets under way.
Ardern was not well enough to chair Cabinet remotely on Monday or to deliver her speech virtually during the lockup for Monday's pre-Budget release of the landmark Emissions Reduction Plan.
A spokesman said Ardern's participation in events this week - including the Budget - would depend on whether she was well enough each day.
Last week while Ardern was isolating but before she tested positive for Covid-19 she chaired Cabinet remotely and took part in Parliament's proceedings.