Gronholm, who leads France's Loeb by eight points after 10 of 16 events, won the first two stages in his Ford Focus.
The two-time world champion is chasing a record sixth New Zealand victory as he seeks to stop Citroen's Loeb taking his fourth straight WRC title.
Compatriot and fellow Ford driver Mikko Hirvonen is in third place.
Tyre choices proved important on the first two stages on the gravel roads through the Waikato district on New Zealand's North Island.
Gronholm and Loeb were on softer compounds, while Hirvonen used tyres less suited to the conditions and was more than 30 seconds behind Gronholm after two stages.
He was 49.2 seconds down in third place by the end of the day's final stage.
Australian Chris Atkinson, whose Ford finished day one in fourth place, had trouble with the balance of his car.
Fifth-placed Jiri-Matti Latvala hit a rock near the end of stage two and finished the day with his Ford's front bumper missing.
Britain's Matthew Wilson, who finished the day in 10th, slid off into a fence on an early stage. He was able to reverse onto the road and continue.
"Fortunately the farmer has built a good fence and that stopped us going too far off the road," Wilson said.
Loeb won the day's third and fourth stages, while Atkinson took the fifth.
