Re-entering after Tier 5 expiry

My YMS visa expires on the 21st of July but I am not scheduled to fly back home to Australia untill the 3rd of August, should I leave the country and attempt to re-enter on a tourist visa, will this be a problem at the border if I have proof of a one way flight to Australia? Can I just stay in the country for the 12 days after my visa expires, will this possibly affect me coming back to England or applying for other visas in the future ?

You should leave before or on the date of your visa expiring. Once your visa expires, you won't be allowed to work, rent, use the NHS etc.

Any reason why you cannot leave before or on 21st July?

Likely you will face problems trying to re-enter as a 'visitor' immediately after your current visa expires.

My partner is coming back to Australia with me amd he is a school teacher so we have had to wait for his school term to end. If worst comes to worst i will just have to leave the country and only re enter the day of my flight to australia

I would not risk any attempt to leave and return as you are contemplating.
You have a 28 day grace period to leave the UK without risk of detention or arrest. In that period you can not work.
My very informal advice is that you stay in the UK and fly out on your proposed date.
Is there any adverse issues to this- YES
If you are intending to return to the UK there is a minimal risk that your minor technical overstay will go against you. It shouldn't really be an issue as the grace period affords a reasonable period to leave the UK post visa expiry.