The opposing party, being the party that recieves the 2nd most number of seats in Parliament, has the mandate to act as an alternative government as well checking the power of the current government through media or in parliament through Question Time.
Arguements For the Mandate
Sections 7 and 24 state the Senate and House of Representatives, respectively, should be elected directly "by the people", meaning that the Opposition is endorsed by a significant proportion of the electorate to fulfil its role and if it did not do so then it would not be representing its electorate;
The opposition also represents a large minority and can claim a mandate on their behalf; and
The role of the Opposition to scrutinise, debate, amend and even block government legislation is an important part of the Westminster system to keep the government accountable and uphold a thorough statutory process, which would otherwise be inable to occur without such scrutiny.
Arguements Against the Mandate
The Opposition did not win the election and, hence, their policies were rejected by the people in favour of the government's. They should, therefore, allow the government to utilise its will of the majority mandate relatively unimpeded.
They do not represent the majority, hence are going against the majority by impeding government business.