One important concept to understand is liquidity: having ready access to your money so you can use it and take advantage of it as soon as possible without having to wait. This is useful for cases where you might have an offer and need to act on it immediately before it expires. This page goes over some tools which allow to to instantly transfer your funds to where they are needed rather than waiting while your funds are held hostage in the system.
Intrabank
If you have two accounts with the same bank (e.g. a transactions and a savings account) they will usually have instant transfers between the accounts. I am not aware of any bank which has a delay for an intrabank transactions. The problem with this is that no single bank offers the absolute best of very product.
OSKO (NPP)
OSKO is based on the New Payments Platform and allows fee-free instant transfers between participating banks. Most banks these days support NPP and you can check if your bank does here. However, it is important to realise that even if a bank supports OSKO, there is no guarantee that a transfer will be made via OSKO and it may be made with the old system which could take 1 – 3 business days. Another thing to note is that some banks place a manual hold on first-time payments or large payments or even for no reason at all (looking at you CBA) and different banks have different maximum transfer limits.
BeemIt
Beemit is a service owned by eftpos that allows fee-free instant transfers between transactions accounts via their linked debit cards. Funds are debited from the transferring account as a Visa/Mastercard purchase and credited to the receiving account as an eftpos refund. This mechanism can be exploited in a certain way which is discussed in Meeting Bonus Requirements. The daily and monthly limits should be noted here.