Depends on the build of the pony - at my yard we have two purebred NF ponies, but one is much chunkier than the other and I think he could take more weight. Ideally, I don't think I'd put more than 9 stone on either of them (maybe 9 1/2 on the sturdier one).
You haven't specified the size of the ponies? You can get a pure NF from anywhere between 12.2hh and 14.2hh ish. They can't be shown above 14.2hh. How big is the pony in question? And is he/she chunky or light?
in a lot of their literature the new forest pony society say a well built, fully grown new forest should carry a stone for every hand they are high.
however, personally probably would not put more than 13 stone regularly on a 14.2hh and only if the rider is a well balanced rider, not a novice.
they are stronger than we give them credit for these natives!
In my book about NF it said they used to have races in the 1900's and the minimum weight for the races was 11 stone with a further 7lb for each inch over 13h.
However it will depend on the build of the pony and also what you intend to do and how fit the pony is. My forester is the old fashioned chunky type built for weight carrying and he has short cannon bones and a deep body. I am only 8.5 stone.
The breed show still have a class for weight carrying New Forests and the riders have to be over 12 stone 7lb to enter and are weighed in the collecting ring!
And here is a photo of my chap - (he was a bit overweight in this photo but has slimed down a bit now but gives you the idea of the sort of build of a chunky NF - he is 13.3