Once-in-a-generation changes to the Senate voting system were followed by the nation's first double-dissolution federal election since 1987.
The poll gave Australia one of its most diverse and divided Senates since Federation.
And at year's end, the confusion in the upper house continues, with the Senate's composition again in doubt.
Following the 2016 federal election, the Coalition had 30 senators, Labor increased theirs by one to 26, and the Greens lost one to fall to nine senators.
The crossbench grew from eight to 11 senators.
That meant the Coalition needed nine extra votes to pass laws and motions.
At the time, Prime Minister Malcolm Turnbull acknowledged he needed the crossbench if he was going to get his agenda through the Senate.
"I'm not going to speculate about negotiations with senators. I'm certainly not going to negotiate in advance. As you know, we don't have a majority in the Senate, so we will be talking to all of the senators and seeking to secure their support for this important legislation. We believe we will win their support."
At year's end, Mr Turnbull appears to have navigated a way through the Senate's rocky shoals.
The Government has managed to pass its Australian Building and Construction Commission laws, its companion Registered Organisations bill, the backpacker tax and most of its blocked welfare changes.
But the Turnbull Government faces a massive struggle to pass its 10-year plan for $50 billion in company tax cuts.
The plan would cut taxes for companies with a turnover up to $10 million a year, followed by cuts for ever larger businesses until 2026-27, when all would pay a 25 per cent rate.
Labor and the Greens support only the tax cuts for smaller businesses.
The Government would have considered its chances in the upper house better than average, with Senators Rod Culleton and Bob Day most likely to vote with it.
However, Senator Culleton, a former One Nation West Australian senator turned Independent, and Senator Day, from Family First, both face High Court challenges to their positions.
If a challenge is upheld, either one could change the delicate balance in the upper house.
The judges of the High Court, sitting as the Court of Disputed Returns, are yet to determine the fate of the two men and their positions.
Of the two cases, the one relating to Senator Culleton is the simplest.
The Constitution says a person cannot sit in parliament if that person has been "convicted and is under sentence, or subject to be sentenced, for any offence punishable by imprisonment for one year or longer."
Senator Culleton had a larceny conviction against his name on election day, which the Commonwealth argues disqualifies him under the Constitution.
He argues that conviction was later annulled and, therefore, should have no impact on his tenure.
If his election is ruled invalid, his replacement in the Senate could well be his brother-in-law, Peter Georgiou, number two on the One Nation West Australian Senate ticket at the election.
Mr Georgiou would take his seat as a One Nation representative in that case, despite Senator Culleton's defection from Pauline Hanson's party.
But Senator Culleton says he feels it will not come to that.
"Like Ned Kelly said, such is life. I just have to see how we go, and put up (with it). But I feel very confident. I don't know how the Government's going to argue annulment."
The Bob Day matter is more complex.
The concerns relate to a potential financial interest he had in his Adelaide electoral office and whether it had breached the pecuniary-interest rules for parliamentarians.
If his election is deemed invalid, a new senator will be elected via countback.
The position would be filled by the candidate next in line following a recount of the South Australian senate vote, likely to be a Labor politician.
Constitutional lawyer George Williams says the court will treat the case with urgency.
"In a case like this, you would normally expect the High Court to act as quickly as possible, as it does with elections and other matters of political sensitivity, because, obviously, one vote in the Senate at the moment could make a big difference."
But it is not just the fate of Senators Day and Culleton creating a political headache for Mr Turnbull.
Speculation is mounting among backbenchers that Defence Minister Marise Payne, Attorney-General George Brandis and Indigenous Affairs Minister Nigel Scullion may lose their roles.
Senator Brandis has faced controversy over whether he sought to favour the West Australian Liberal state government over the Commonwealth in a court case relating to the Bell Group.
He has also been embroiled in claims he sought to damage the Office of the Solicitor General.
Opposition Leader Bill Shorten has told his Labor caucus he believes Senator Brandis will not be returning in the New Year.
"I don't think it's a matter of if the worst attorney-general in Australia resigns, I just think it's a matter of when. I don't think Malcolm Turnbull has the guts to sack the most accident-prone and incompetent minister in an accident-prone and incompetent government whilst parliament sits. But as soon as the Christmas turkey goes in the oven, George Brandis will be cooked." (laughter ...)
But Malcolm Turnbull is refusing to be drawn on the issue.
"I have an excellent ministry. I'm delighted with the hard work my colleagues are undertaking, and it is measured in results. And the results are building up -- we are delivering. "
The speculation comes amid pressure on Mr Turnbull to return former prime minister Tony Abbott to a frontbench position, something the Prime Minister says he will not be doing.
But it might be Mr Abbott's prime-ministerial ambitions that could cause the biggest headache for Mr Turnbull.
Some backbench Coalition senators are believed to be actively agitating for a return to Mr Abbott.
But Defence Industry Minister Christopher Pyne says there need be no such concerns because the question has been settled once and for all.
(Presenter:) "Tony Abbott and Malcolm Turnbull despise each other."
(Pyne:) "Not true."
(Presenter:) "Are you serious?"
(Pyne:) "Absolutely serious! They are very old friends. Of course, they have a difference of opinion about who the prime minister should be. That's been played out in public. We know that. And Malcolm Turnbull is the prime minister."
What is certain, however, is the Senate is likely to have its third new look in just over 12 months when parliament returns in February.
Just what that look will be remains to be seen.
