The hay will compost in situ, and compost reduces in volume at a dramatic rate. A ~5 y.o. bed I have that is ~28" deep still loses several inches a year, though its slowing down as sand washes in (its on a hill, the uphill side is at ground level) and the stuff at the very bottom is finally reaching the point were it won't reduce as fast.

With 30" to fill, I'd at least look at putting a lot of sand or sandy "topsoil" in the bottom 12"-18" with the hay as more stable "fill" (actually, I'd use the cheapest dirt or clay I could find for the bottom 12"). For veggies, no roots will reach that far down anyway, and even if they do whatever is down there will improve quickly with nutrients leaching down from above.

Thanks for the fast reply. So I gather from your reply that the hay would be ok but the volume reduction would be a problem. Would I see this in the initial growing season or would it take longer? I was aware of this problem but was only hoping to spread the cost out over 2 or more seasons. In other words after the garden has ran its course this year add additional mulch, compost, topsoil as required to bring the bed back up to the required level. Or I could just take your advice and use the cheapest 'sand' I could find. What about the soil I removed from the area before placing the bed? It has grass and weeds in it; could they be a problem when buried so deeply?

Thanks again,
Wormsign

The removed soil would be a great base, toss that down there and cover it with a couple layers of cardboard. That is probably even enough depth to stop bermuda grass.

If you are putting in entire bales of hay still tightly packed, the volume reduction would take longer, but if you split it up first, it would be dramatic within the first season.