I took my phantom 3 advanced for its first fly today at about 15 ft, i left it hover just to see how stable it was (i had completed a calibration).
The phantom, every so often would drop in altitude about 1-2 ft. It wouldnt really sway left or right, just altitude. It kept happening until it nearly reached the ground!
I wasnt touching the controller.

The battery was over 80%, the controller was fully charged, latest firmware on phantom and controller.

Is this common? This didn't happen with my P1,P2,P2v+ or my inspire1!

Please help!

Can't really think of a reason really.. Its definetly not "normal" but not out of the ordinary. I see you live in Australia so its possibly hot ( well hotter than the UK anyway ). All I can think is something it making the barometer fluctuate as you say it only goes down not side to site the GPS is good. The phantom takes baseline readings at power on I believe so maybe its detecting changes in pressure,wind,temprature.
Yu dont say the area you are flying in, remote location, urban, city. Try it in differnt locations ( keep in near and low ) and see if it continues to do it.. Also make sure VPS is ON, although at 15 feet its not having much influence.

Remember to buy extra propellers and one or two spare batteries to keep you flying

www.Phantom-Guide.co.uk - The Ultimate Phantom 3 Resource

may be a limited fly zone....

Hi, my Phantom 3 professional behaved the very same way the first two or three flights. It would drop in altitude without  any input on the controller. The problem resolved itself.

Re- calibrate your compass

Ken, what is your firmware version? The latest? Do a IMU calibration first, it is related to the barometer.If you use the latest firmware but still drop altitude very often. You can contact support and sent them your MC data (log) to analyse.

Mine did the same for the first few flights. The issue went away after I did the IMU calibration as suggested above.