Hi Mark. Thanks for your question.
Each year the government presents a plan for their spending to the Parliament. This is called a budget but takes the form of bills called supply bills. Supply bills allocate money to the government to pay employees and keep services running. If supply is blocked then the government may not be able to maintain all services and may have to stop paying some government employees.
In 1975 the opposition deferred voting on supply (or money bills) in the Senate in an attempt to force the Prime Minister Gough Whitlam to call an election for both the Senate and the House of Representatives. In response Mr Whitlam decided to ask the Governor-General to call a half-Senate election to resolve the situation. However he was dismissed before this request could be formally agreed to.
More detailed information on the Dismissal can be found here.