Altium Power Plane Gerber inverted
Power planes are always created as negatives. You have a couple of options:


Specify to your manufacturer that this layer is a negative. Many manufacturers will ask you the polarity of the layers. 
(My personal preference) Instead of using planes, use polygon pours. This way there is no doubt, and it's clear where the copper is. 


The choice is ultimately up to you as a designer, but I tend to find #2 easier to do and it doesn't rely on having smart people at the board house.