Canterbury seamer McEwan took career-best figures of 5-20, while part-time bowler Stewart snagged 3-17 from 10 overs as Northern capitulated.
Northern will have to hope they can bowl Canterbury out cheaply tomorrow (Fri) as they search for victory, while the Cantabrians will look to dig in and set ND a big target to win.
At the Basin Reserve, Auckland built a steady platform after they lost the toss and were invited to bat by Wellington on day one of their encounter.
Auckland No 3 Anaru Kitchen made 95 as he laid the groundwork for a big total for the visitors, while Colin de Grandhomme carried on the momentum late in the day with an entertaining knock as Auckland closed play on 319-5.
The former Zimbabwe under-19 player and part-time New Zealand international finished the day unbeaten on 81 but managed to blast a cover drive straight in to an unsuspecting bird, while one of his sixes also broke a window in the Long Room of the RA Vance Stand.
The 26-year-old will resume the hostilities tomorrow with Dusan Hakaraia at the other end, who finished with eight next to his name this evening.
Seamer Mark Gillespie was the pick of Wellington's bowlers as he grabbed 2-48 from 19 overs, while three other Firebirds took one scalp each.