It appears clear that the Governor-General (acting on the advice of the Prime Minister as per Westminster convention) can under Constitution section 5 prorogue the current Parliament and then appoint a new session to commence on 18 April. Presumably that is what occurred this morning by Proclamation pursuant to section 5. This is a reserve power at least in some circumstances, so the Governor-General probably could if he chose have made up his own mind about it rather than simply taking the advice of the Prime Minister. It would appear that he chose not to do so and simply took Turnbull’s advice.
In that sense one might think that Turnbull has outflanked the Senate resolution of last Friday not to reconvene without unanimous support. However, as ABC psephologist Antony Green tweeted this morning, while the Governor-General’s proclamation under section 5 can require both Houses of Parliament to reconvene on a specified date to commence a new session, as far as I can see neither section 5 nor any other constitutional power of the Governor-General would prevent the Senate from immediately resolving to adjourn until a later date in the newly appointed session of Parliament (say after 11 May when a double dissolution is impossible).
However that too might be thwarted by the government with the Governor-General’s concurrence. It may well be that the very step of the Senate immediately adjourning on 18 April would constitute a failure to pass the ABCC bill within the meaning of Constitution section 57 (the deadlock/double dissolution provision) such as to allow Turnbull to advise the Governor-General to dissolve both Houses and call a double dissolution election in any event.
There’s never been a more exciting time to be a constitutional lawyer (except 1975 – but I was only a young lad then – and …).