A method for injection of a lift gas into a wellbore production string comprises determining production pressure within the production string, and autonomously controlling a variable orifice gas lift valve in accordance with the determined production pressure, wherein the variable orifice gas lift valve controls the injection flow rate of the lift gas into the production string. A valve comprises a housing defining an inlet, an outlet and a flow path therebetween, and a valve member linearly moveable within the housing between first and second positions to vary flow along the flow path, wherein the valve member is prevented from rotation relative to the housing during linear movement between the first and second positions. The valve further includes a rotary drive and a transmission arrangement interposed between the rotary drive and the valve member for converting rotation of the rotary drive to linear movement of the valve member.