The short term money market is more accurately named exchange settlement accounts (ESAs). These are bank accounts for banks which much be kept with the RBA, and must always be in credit, to 'settle' the flow of money that is passed about through cheques.
say a commonwealth bank client, bob, gives larry, a NAB client, a cheque for $100. Larry cashes the check with his bank and has to wait three (3) days to be paid (while the banks work their magic)... in the end Larry's money is moved from the Commonwealth bank's ESA to the NAB's ESA. You'll notice that liquidity has not changed, there is still just as much money out there it has just moved places.
This is how the RBA limits the supply of money (liquidity), since they can easily buy and sell govt securities and bonds to banks without much hassle and at the same time affect and influence interest rates (which is thanks to the forces of demand and supply in the money market).
Say the RBA takes a loose monetary stance. They want to lower interest rates and to do this they must increase the supply of money (liquidity). Say the Cth bank has a Commonwealth Govt Security worth say.... $100 (unrealistic, but play along, okay?) @ 5% for a period of 10 years. i.e. treasury owes the bank $100m plus interest after the 10 years is up. The RBA would say to the Cth bank "we'll pay you $170 for that security!" The Bank would do its sums and find out that they're getting a tiny yield of 2% of their original investment, but agree anyway because they're still making money. The RBA would write a CHEQUE for the $170 and give it to the Cth bank, who now have $170 more in their exchange settlement account. They therefore have more money in circulation, and the RBA is hoarding less.
To increase interest rates, the RBA would sell old Cth Govt securities cheaply. So say the RBA has a security worth $100 @ 4.25% over 12 years, they may sell it for $150 (if they were insane). At any rate, the banks like to make a profit, so the Cth Bank might buy it, and in doing so they will write a CHEQUE to the RBA who will debit the Cth Bank's ESA by $150. Thus, money has gone out of circulation; a decrease in supply and ceterus paribus a rise in price.