Studies employing nonlinear interactions of ultrashort THz pulses with matter are becoming a promising scientific research field enabling access to ultrafast dynamics unattainable by other methods. To enter this regime, THz pulses with energy ranging from hundreds of μJ to the mJ level are necessary. However, techniques that provide pulses with such energy levels are still not widely established. Upscaling methods of laser-solid interaction is limited by the damage threshold of materials, while the mechanism of THz generation from high-intensity laser-gas interactions is not fully understood yet. Here, we establish the photocurrent model of laser-driven plasma THz generation in the unexplored high-intensity regime by accounting for high-ionization states of the target gas. Our model shows excellent agreement with experimental observations, providing a clear explanation of phenomena in both spectral and temporal domains. Moreover, in such a regime it explains a newly observed significant increase in conversion efficiency from laser to THz. In the experiments, we achieved a generation of 0.2 mJ THz pulses, driven by a Ti:sapphire laser with a conversion efficiency exceeding 1 %.