I wouldn't advise using current for calculating torque when you know the KW (HP) and speed.
Given 300KW, Hp would be 300/.746 or 402 hp. Assuming a nameplate speed of 1770 rpm, torque in ft-lbs would be
torque = hp x 5252/rpm = 402 x 5252/1770 = 1193 ft-lbs
Now its important to understand that nameplate data on a motor is FULL LOAD DATA. Therefore, when this motor is loaded to 1193 ft-lbs torque, the speed will sag from 1800 to 1770rpm and the calculated hp will be 300kw or 402 hp and the amps should be whatever the nameplate FLA or FLC says it is.
Don't know what you might have meant by no load torque---no load means zero torque.
Now, to get starting torque, you must consult the speed/torque curve for the motor. In the United States, most motors are designed to NEMA Design B specs which pretty well defines the torque/speed characteristic curve from no-load down to stall. What you might have in Australia is unknown to me but very important for you to understand.
If this motor is on an inverter, then you must use the same speed/torque curve but you must identify precisely where on the curve you are operating under starting conditions. Normally, you would not stray very far from the full load operating point on the curve due to the limited ampacity of the inverter and the ability (hopefully) of the inverter to manage output voltage and frequency properly to keep the motor operating in near-synchronous mode.
Going back to your original question, if you are starting across-the-line, you can expect to see the 5x nameplate current you mentioned but, unfortunately, the torque is far less than that. The problem is that, on start, an induction motor is standing still (equal to 0Hz) and is hit immediately with 50 or 60Hz. This places the motor far outside its synchronous operating range and its ability to convert amps into shaft torque is severely impaired. This confused state inside the motor exists until the rotor struggles up to typically 90% or so of its sync speed where it transitions to near-synchronous operation. When that happens, the amps fall dramatically, efficiency improves dramatically as well, and the expected correlation between motor load and amps occurs.
Exactly how much torque the motor can develop when operating dis-synchronously is entirely a matter of design compromises built into the motor. That's why a speed/torque curve is essential to answer your question.