There is actually no set formula. It depends upon the kind of foam you are using.
If it is a drop in seat, the seat pad should drop into the wooden frame. In such a case, the foam must be of the size of the frame or the board that it's going on. Anything extra means the seat pad wouldn't drop into the space available. With the drop in seats, the cover is fixed with tacks to the frame/board. If it is the other kind of seat pad where fabric is fixed to the frame or board, then we suggest cutting the foam slightly bigger than the frame or board so that the edge of the board or frame is not felt through fabric.
In all the other cases, finished cushion must be bigger than the cover it's going in to. If your foam has a polyfibre wrap on it, you can cut it the exact size of cover and the wrap will offer the extra size that is needed to make the cushion filled. But if your foam doesn't have a polyfibre wrap, it must be cut a bit larger than the size of the cover. Softer the foam, more may be added to measurements of cover size.
An example will make this clear. If there is a cover measuring 18" x 18" x 2", order the foam of the same size and order 4oz polyfibre wrap on foam or else order a foam of 18.5" x 18.5" x 2.5" size if there is no polyfibre wrap. Cushions, which have this wrap have more rounded look on corners rather than plain foam that has a more block/squared look. However, any one of them is fine. It is only a case of which one matches with your style of furniture.
Just be aware that no foam can make your cover larger than it is when stretched but if you order the foam just the size of covers, it may look baggy.
You can definitely send the suppliers of foam in Northern Beaches your covers to be cut to any size that you need. However, the above-mentioned hints must help you make the right decision regarding the sizes that you should order.