I guess it depends on what the substitution is for.
Certainly if the butter is just for flavour, it's a reasonable substitute (I think I'd use slightly more yogurt).
But 9 times out of 10, fat is the main reason the recipe is calling for butter!
Yogurt cannot substitute for a fat (butter) because it has very little fat.
If you lower the fat content of a baked good then you'll generally end up with a product much denser and less flakey.
So if you feel you must do this for some reason, then I would increase the sugar in the recipe to compensate (sugar inhibits gluten formation and will help to make the result less chewy).
The other thing you'd better keep in mind is that yogurt is naturally sour and will become even sourer when baked at a high temperature.
The longer you bake, the more sour it will become, so you might need to amp up the sugar significantly to maintain the proper flavour.
So for the most part, no, yogurt is really not a substitute for butter or any other fat.
Obviously this is partly a matter of personal taste, and if you're happy with the result, then continue doing what you're doing -
but I think I'd definitely hesitate to eat a cake that was baked with yogurt instead of butter.
cooking.stackexchange.com/users/41/Aaronut
I'll admit, I'd never tried it, but for muffins, I'll often replace 1/2 to 2/3 of the oil in a recipe with some sort of liquid-like fruit (either applesauce or mashed previously frozen banana), so I don't see why yogurt wouldn't work.
In those case, the oil went in as a liquid; if you had a baked good that required creaming the butter, or cutting it in, I would not try replacing it.
I would also be concerned with the increased acid; it's possible that you might want to experiment with cutting some of the baking powder and replacing it with 1/2 as much baking soda as you removed, so you're more pH neutral, but I have no idea how baking soda is needed to balance out yogurt.
As you're from NZ, I'm going to assume 'biscuits' in this case is what Americans call cookies -- I would not try replacing butter in what Americans call biscuits, as you add the fat solid (sometimes cold), to make the them flaky.
cooking.stackexchange.com/users/67/Joe
I do it all the time.
Usually I swap out half the butter for half as much yogurt, either plain or vanilla, usually low fat.
Sometimes I do Greek yogurt.
I'm working on adjusting certain recipes so that it's ALL yogurt
and I've had the best luck with banana bread and muffins.
They are delicious and nobody knows the difference.
If a recipe has more than 1/4 cup of butter, I definitely start swapping.
I do the same for oil
but it's closer to an even swap.
cooking.stackexchange.com/users/12626/Laurie
I substituted equal parts of yogurt for butter in my chocolate chip cookies and the structure of my cookie was completely flat like a pancake (possibly flatter with holes in the edges).
But when you use your spatula to remove the cookie you can only remove the inside of the cookie so it created a sort of cookie dough ball look.
I was terrified but when compared to my normal cookies (made with the butter) they were ranked the exact same in my food science experiment in my college class.
So taste wise my cookies were still delicious but appearance wise
they were a little hideous.
P.S.
I used the chocolate chip cookie recipe from smitten kitchen (I love them!).
cooking.stackexchange.com/users/8255/Taylor
It works awesome in banana bread, especially if you use vanilla yogurt.
cooking.stackexchange.com/users/8966/Christina
By baking, I mean cakes / muffins / biscuits.  This is mainly to cut down on fat.  I've tried replacing butter with yoghurt in all of these and the results were OK.  Are there any cases where it's not a good idea?  I generally replace with equal ratios e.g 100 ml butter = 100 ml yoghurt.  Is this the best ratio?
In baking, can yoghurt replace butter?
cooking.stackexchange.com
<substitutions><baking><butter><yogurt>
cooking.stackexchange.com/questions/3644
cooking.stackexchange.com/users/812/rbrayb
Partially or fully substituting with plain or vanilla yoghurt can work in some recipes like banana bread, but the low fat content makes it a poor substitute in other contexts. Adding sugar will inhibit gluten formation and make the result less chewy. Replacing all the butter would be too extreme as the recipe would be missing the fat content. You could try swapping half the butter for yogurt.
Yogurt has very little fat compared to butter so try increasing the sugar to avoid the result being very chewy.  Swap half the butter for yogurt. It works especially well in banana bread.