About 2500lbs is all it could take if they had 2 crew, as you said Gen Ties. I don't know the weight of the aircraft at Gambier but the medical fitout in the B200 usually brings the operating weight up to between 4-4.5 tonne. Weight is a big issue with these aircraft.
My rough calcs came up with a nil wind, no holding fuel figure of around 2300lbs
What was the weather in Sydney? I don't know, but if there was any kind of weather holding etc. they may have had to make a stop for fuel along the way, which would have been out of the question considering the urgency of the job.
I don't know if this is the case but also consider that the patient may have been quite ill at the time. Maybe requiring additional, fairly weighty equipment to be attached to he or she. This can add up.
Flight nurses can handle many different patient conditions but quite often, a doctor may be required as well. Extra weight again.
You hopefully can see now all the variables that can come into play on different jobs. I can't recall if the Gambier patient required all of the support i've mentioned but if so, the job would not have gone with 2 pilots on board.
As a side note, 2 pilots had no positive outcome at Lockhart River.