The experienced Otago seamer took seven for 107 to help dismiss Northern Districts for 355 at Bay Oval in Tauranga yesterday.
Otago slumped to 65 for three in reply, but rallied to reach 190 without further loss at stumps on day two.
Dean Foxcroft is undefeated on 76 and the in-form Thorn Parkes is at the wicket with him on 45.
They have put on 125 and will need to push on if the Volts are going to strengthen their position.
Duffy has done his part well. He started the day with five and added another two to his growing haul of wickets.
Otago coach Dion Ebrahim was not surprised his spearhead has hit the pitch running.
"He has definitely made some adjustments over the winter and it is probably a testament to the amount of cricket he has played over the last year."
Duffy played for the Black Caps and New Zealand A during the New Zealand off-season and the extra workload has clearly sharpened his game.
"He has stayed fit, he has stayed healthy and he is in a good space."
"He’s gone back to what he does best and that is swing the ball. He has always had a high accuracy with his stock ball.
"And what has also assisted is the accuracy and consistency of the other three seamers.
"It is often partnership bowling that gets the bulk of wickets. Jake has been, I guess, the fortunate one."
Northern scored a few more runs than the Volts would have liked, though.
Kristian Clarke (52) and Neil Wagner (41) added 86 for the eighth wicket. That hurt.
Duffy eventually helped snuff out the resistance. He knocked Clarke’s leg stump out of the ground and trapped Joe Walker lbw.
Duffy took eight wickets in the 168-run win against Auckland last week and has 15 in total so far this summer — that is two more than he managed during the entire Plunket Shield campaign last season.
If he was able to continue taking wickets at the same pace, he would finish the Plunket Shield with 80 wickets.
That will not happen, but it has been a special start.
Ebrahim was awfully pleased to see Foxcroft in the runs as well. The South African-born right-hander missed two years of cricket due to Covid.
He came in at a tricky time but got set at the crease and will resume this morning with an opportunity to post his maiden century in the format.
The 24-year-old averages more than 50 in list A cricket, but has not enjoyed the same success against the red ball.
The other major plus to emerge has been the form of Parkes.
The 22-year-old left-hander scored back-to-back half centuries against Auckland and is five runs away from a third.
Central Districts dispatched Auckland by an innings and 99 runs at Eden Park Outer Oval. The home team was routed for 75 on the opening day and only managed 187 in its second innings.
Tom Bruce scored a big hundred for Central and Doug Bracewell helped lay the ground work with a five-wicket bag.
Wellington is struggling against Canterbury at the Basin Reserve. It is nine for two in its second innings and trails the visitors by 368 with two days to play.
PLUNKET SHIELD - Otago v Northern Districts
Northern Districts
J Raval c Chu b Mckay 29
H Cooper c Cumming b Muller 112
B Popli c Cumming b Duffy 12
J Carter c Chu b Duffy 6
T Seifert c Phillips b Duffy 5
C de Grandhomme c Chu b Duffy 52
S Kuggeleijn b Duffy 8
K Clarke b Duffy 52
N Wagner c Chu b Rippon 41
J Walker lbw b Duffy 15
M Fisher not out 4
Extras (8b, 1lb, 8w, 2nb): 19
Total (in 86.2 overs): 355
Fall: 1-77, 2-109, 3-123, 4-144, 5-230, 6-242, 7-242, 8-328, 9-351, 10-355.
Bowling: J Duffy 30-6-107-7 (2w, 2nb), M Rae 19-6-55-0, T Muller 23-5-100-1 (1w), J Mckay 12-1-75-1 (1w), M Rippon 2.2-0-9-1.
Otago
H Rutherford c Seifert b Kuggeleijn 9
J Cumming c Raval b Fisher 24
D Phillips c Raval b Fisher 23
D Foxcroft not out 76
T Parkes not out 45
Extras (2b, 3lb, 8w): 13
Total (for 3 wkts in 74 overs): 190
Fall: 1-11, 2-48, 3-65.
Bowling: N Wagner 13-3-30-0, S Kuggeleijn 8-2-33-1 (2w), J Walker 17-1-41-0, C de Grandhomme 11-2-19-0, K Clarke 10-2-22-0, M Fisher 10-0-28-2 (2w), J Raval 5-0-12-0.
