The closeness and ferocity of the battle mean the title could go anywhere, with other riders such as Christchurch's James Smith, Wellington's Sloan Frost and possibly even visiting Australian Linden Magee also looming as title contenders.
Charlett had enjoyed a fantastic opening round at Ruapuna, Christchurch, on January 11, but it was a different story for the Suzuki ace at Timaru's Levels Raceway a week later as fellow Suzuki rider Hassan mounted an attack.
Hassan qualified his Suzuki GSX-R1000 fastest at the Levels circuit and then proceeded to score an impressive hat-trick of wins, elevating him from third - and a distant 40 points behind Charlett - to second.
In contrast, Charlett finished 3-3-5 in his three outings at Levels.
Jaden Hassan (Suzuki GSX-R1000) has taken huge chunks out of the defending champion's superbike class lead.
Hassan, from Westmere, had been expected to be one of the superbike class frontrunners this season after his scintillating performances in the pre-season Suzuki Series, but nobody expected such a rapid resurrection.
"I'm pretty stoked with how things have turned out now," said Hassan.
"I was pretty disappointed after last weekend, so to come out on top this weekend is a great feeling.
"I'll have a couple of weekends off now and then do some testing to get set for the next round at Hampton Downs. We have a six-week break now, before wrapping it up with two rounds back-to-back.
"This is the best South Island racing experience I have had. It was good for me to be as quick as Dennis on his South Island tracks and now he'll come north to hopefully be chasing me on the North Island tracks. That's the plan anyway."
Now third in the superbike standings is Smith (Honda CBR1000RR), eight points behind Hassan, with Frost (Suzuki GSX-R1000) in fourth spot in the championship chase and transtasman visitor Magee (BMW S1000RR) rounding out the top five.
Jaden Hassan.
Meanwhile, in the 600cc Supersport class, Christchurch brothers James and Alastair Hoogenboezem lead the way on their identical Suzuki GSX-R600 bikes.
James led the class after the opening round but Alastair came on strong at Levels, taking pole position in qualifying and then scoring a lap record on his way to winning all three races at Levels.
James crashed in one of his races but recovered to finish 13th. He is just 16 points behind his brother.
Christchurch's Cameron Hudson (Yamaha R6) remains third overall in the class, 17.5 points behind James Hoogenboezem, with Kaitikati's Rhys Holmes (Yamaha R6) and Manukau's Toby Summers (Yamaha R6) rounding out the top five.
Leaders in the other classes after round two are Orewa's Avalon Biddle (Superlites); Whangamata's Ben Rosendale (Pro Twins); Blenheim's Tim McArthur (250cc production); Christchurch's Sam Davison (125 GP) and Auckland pair Adam Unsworth and Stu Dawe (sidecars).
Round three is set for Hampton Downs on March 7-8, with the fourth and final round set for Taupo on March 14-15.