An audacious declaration from South Australia's Jake Lehmann and a career-best score for NSW's Ryan Hackney has brought the final-round Sheffield Shield match at Karen Rolton Oval to life on day three.

On a docile pitch that had produced just 16 wickets in eight sessions, Lehmann declared South Australia's first innings closed at 7-309, 138 runs in arrears of NSW.

NSW took up the baton in the final session thumping 2-203 off just 42 overs for a lead of 341.

Hackney led the way in his fifth Shield match, ending the day unbeaten on 98no.

After also scoring a half century in the first innings, he struck 13 boundaries and a six in his 115-ball knock.

Along with fellow inexperienced opener Blake MacDonald (61), the pair added 143 for the first wicket.

Ollie Davies helped the scoring along in the final half-hour with a whirlwind 31 off just 24 balls.

Earlier, South Australia scored at just over three runs an over before declaring when their 100-over bonus point period ended.

Lehmann impressed with an unbeaten 101, clocking up 13 boundaries in his 178-ball stay at the crease.

Daniel Drew was more sedate during his four-and-half hour innings of 85.

Experienced paceman Chris Tremain and first-gamer Ryan Hadley both picked up 3-75 for the Blues.

The inexperienced NSW side must collect victory in Adelaide to avoid a winless campaign for the first time in a 10-match season, while South Australia are seeking to hold off Tasmania and finish fourth.