I have used the Christmas Pavlova recipe many times with great results. Now it is marshmallow all the way through. I would have thought the oven is to hot or cold but I don't seem to be able to work out which way is right; or is it something else all together? The weather is very dry and hot but this is not unusual here. Sandy
A pavlova should have a crisp, dry outer shell but the centre should be soft and marshmallowy, and not crisp. However We are not sure from your description if the outside shell of the meringue is soft too. If you are cooking in humid conditions then sometimes it can be difficult to cook meringues, but as you are in hot, dry conditions we do not think the atmosphere is the problem.
Nigella cooks her pavlova meringues by heating to oven to 180c/350F and then reducing the heat to 150c/300F. This early blast of heat helps to set the outer shell, but leaves the inside still soft. It could be that your oven is running a little too cold, so we would suggest, if possible, that you check your oven temperature with an oven thermometer. These are realtively inexpensive and can be bought at many cookware stores and department stores.
The cornflour (cornstarch) in the meringue mixture helps to give the marshmallowy interior. If you wanted a chewier interior then you could try omitting the cornflour from the recipe.