Scott Morrison has rolled the dice on an election victory in May next year, underpinned by an early federal budget that will predict a return to surplus in 2019-20, a year sooner than forecast.

But his attempt to regain control of the agenda was spectacularly knocked off course with the decision by disgruntled Victorian MP Julia Banks to quit the Liberal Party and join the crossbench, making the Coalition's hold on power even more tenuous.