Usman Khawaja hit an unbeaten half century in trying conditions as Queensland reached three for 121 at tea on day one of the Sheffield Shield match against South Australia at the Gabba.

With Test selector Rod Marsh watching, the stylish left-hander was patient in reaching 71 not out on debut for the Bulls, punishing the inexperienced Redbacks seam attack when they erred in length.

Khawaja, who moved from NSW in the off-season, brought up his half century in style moving down the wicket and crashing Redbacks skipper Nathan Lyon through the covers for four.

With overcast conditions and a green wicket, Lyon sent the Bulls into bat and received early reward when Wade Townsend fell leg before without scoring to Chadd Sayers.

Khawaja and Andrew Robinson steadied the ship before the latter edged Gary Putland behind for 30, with 69 on the board.

Another Bulls Test hopeful Peter Forrest progressed slowly to 16 before edging Joe Mennie to Michael Klinger at first slip. Joe Burns was the other not out batsman on two.

The first session was interrupted by a 54-minute rain delay after which Khawaja, with 11 to his name, received a reprieve when Putland dropped him at long leg off the bowling of Sayers.

Otherwise the 25-year-old batsman, who made the last of his six Test appearances for Australia against New Zealand in Hobart 10 months ago, has looked relatively untroubled.