Small platform for a small water tank

I need to pretty up things an I want to integrate my firewood store into a space under the water tank.
It's only a kilolitre so when full about 1200kilos. Pallet sized as usual
It will abut our undercover area and I want to be able to use the space under the tank to store firewood and have access from the brick paved area.
The posts in the attached picture are 1200mm apart, 1100mm C2C with footings 300 * 300 and a depth of 600mm.
This is plenty to hold up the very small roof above the area but is it strong enough to put a ledger on and use the posts as part of the support for the water tank. Ditto a question on the other footings, is 300*300 and 600mm deep enough.
I'd want the tank about a metre off the ground and I'd use 12mm threaded rod as cross bracing as I have a good few lengths here
300 * 300 and 600mm deep is the requirement for the house stumps when we had the block soil tested before restumping

Those footing sizes for the house and verandah are as much intended for hold down, as hold up. They'll be fine to hold up the tank at each corner (just 300kg each) also considering you plan to brace it well.

Good-Oh
I'll head out to the big green shed on Friday then and get the stirrups and the post and ledger materials and bags of concrete mix.
We are ripping up all that brickwork and having it redone, the storms of a couple of years back weren't exactly kind to the sand base. Paying a young bloke to redo it all and extend the area a bit.

OK While that size footing might be OK for the IBC we may decide in the future to go with a bigger tank in the same spot.
So I just started digging out around the posts [ yes I supported the structure] and I'll lay a 100mm pad underneath both post footings, if I can find a few 50mm bar chairs or something similar I have a scrap or reo here that will also go in the hole. I'm covered for the future then, just in case.
Might take me an extra day to dig it out if I take it slow and easy

I was hoping to get this finished today but with all the rain we had last night and some of the hole collapsing I think it will be Wednesday before I can mix the concrete and do the solid base.

OK Seems like this project went West but I did get back into it over the last few days.
Only one post in the ground so far. But I did something a bit different. I have all these home made pavers my dad made decades ago, but only 40mm thick so not load bearing. I got some construction glue that was very close to its use-by date and I glued 3 of them together with an excess of glue and left them to cure for a month. I dug that 600mm deep hole and wet it well and pounded the combined slab down hard and then rammed earth around the H4 post. I know that 250*250 is a bit on the small side for a bigger tank but should be fine for a tonne. I figure if we wind up with a bigger tank I can easily add one more post in the middle. I am going to buy a 140*45 for the ledger just in case [ the base is 1200*1500 but I'd rather go bigger than minimum] and I'll use as many joists as needed to support a full tank, thinking 6 minimum

OK So I went to TBGS and bought some 70*45 H3 on the advice of my mate and did 4 T-section joists at 330mm centres.
70*45 on the edge and the 70*35 over the top.
I'll probably just use plinth board for the decking.
It's far from perfect; despite my best efforts to get it true and plumb things moved and it is not square.
What it is tho is very solid

Well it's finished.
Planked it with H3 150 * 25s
Mate wants me to put a couple more bugles on each side as he thinks that the 200kilo rating of a batten screw is optimistic. Next job is a chicken coop to hold a dozen birds.