Transmission not shifting properly?

G'day guys, I have a 2003 wh v8 statesman. Just recently the gearbox seems to not be shifting correctly. It only seems to be 1st gear that is having issues. So, it won't change out of first until I get to about 3,000 RPMs, then I have to take my foot of the accelerator then it will shift fine. The other gears seem to be changing correctly. If the car drops below 50kph it will change back to first. There's no noise nor is than any shuttering everything is still smooth. Any advice would be appreciated. Cheers