Robertson said the government already made a decision to significantly spend more in Budget 2019, and in that way, the government has already recognised the issue of a slowing economy.
That budget only took effect from July and plans $10 billion of new capital spending and transport spending of $17b over four years, which will have a significant effect on the economy, the minister said.
"There is a significant amount of stimulus in that budget," he added.
"Some of the issues we had faced were through capacity constraints and we've been working through those, particularly with infrastructure," he said.
Robertson did not rule out announcing additional spending in the half-year budget update expected in December.
He acknowledged that monetary policy under Reserve Bank governor Adrian Orr and fiscal policy must work together. He indicated that New Zealand is in "uncharted territory".
"I think Adrian's quote was 'we live in very very very interesting times'. And that's a challenge for him."
The finance minister noted that other countries around the world have worked with negative interest rates for some time.
"The way it is played out is varied. It doesn't mean they all become worried and concerned about people putting money under the bed, because it doesn't get passed onto the run of the mill customers."
The good news for New Zealand, Robertson added, is there is still more room for rates to fall before heading into negative territory.