The West Australian Parliament looks set to legalise voluntary euthanasia within days, with MPs expected to strongly back the bill.

Key points: Upper House MPs were expecting an all-nighter, but a breakthrough came overnight

Upper House MPs were expecting an all-nighter, but a breakthrough came overnight Those MPs still need to vote on the proposed laws, but this could happen as early as today

Those MPs still need to vote on the proposed laws, but this could happen as early as today If they support the bill, it will still need to be voted on by to the Lower House of Parliament

The development came after a late-night parliamentary sitting brought the process to the verge of a conclusion, after weeks of prolonged debate — particularly in the Upper House.

Upper House MPs are expected to vote on the voluntary assisted dying bill as early as today, with supporters confident they have the numbers to ensure WA becomes the second Australian state to legalise voluntary euthanasia.

If the bill does pass the Upper House, it would have just one more hurdle to clear, with the Lower House to be recalled next week to approve more than 50 of the Legislative Council's amendments.

But that process is not expected to cause any headaches, meaning WA's VAD bill looks set to pass by mid next week.

Voluntary euthanasia supporters held several rallies at Parliament. ( ABC News: Hugh Sando )

Opponents of voluntary assisted dying laws also held rallies to voice their concerns. ( ABC News: Hugh Sando )

It would then be at least another 18 months until terminally ill patients could access the scheme, due to a lengthy "implementation period".

The Government's Upper House leader Sue Ellery said the progress was an important step but stressed the battle was not over.

"I'm not going to count my chickens before they are hatched, we have still got a job to do," she said.

Threats of all-night sittings

The looming final vote follows an at-times heated debate, with more than 170 hours in State Parliament spent debating a bill which became particularly bogged down in the Upper House.

The slow progress, amid accusations against Liberal MP and prominent euthanasia opponent Nick Goiran that he was filibustering, led to the Government ordering extended sittings and threatening all-night sessions this week.

MPs arrived at State Parliament on Wednesday with pillows, sleeping bags and swags in anticipation of an all-night session, which some were expecting to continue until after sunrise on Thursday.

But the Upper House made much more rapid progress than it had previously in debating the bill line-by-line, with proceedings wrapping up at 1:00am.

One Nation MP Colin Tinknell came to work expecting an all-night session. ( Supplied: Colin Tinknell )

Liberal MP Steve Thomas, an opponent of the bill, said the avoidance of an all-night session was for the best.

"Everyone is going home a little tired but hopefully not at a dangerous stage," he said.

"This bill has received significant scrutiny and I think we have left it in a better place than it was."

The Legislative Council agreed to more than 50 amendments even though the Government had repeatedly insisted none were needed, which critics said showed Labor had been careless with the legislation.

Premier Mark McGowan introduced the voluntary euthanasia bill in August. ( ABC News: Manny Tesconi )

But Ms Ellery said none of the amendments agreed to would alter the substance of the bill, with advocates having previously feared the Upper House could make wholesale changes.

"We took the principle that if it did no harm to the bill and it helped us get the legislation through we would accept amendments," she said.

The proposed WA voluntary euthanasia scheme would allow terminally ill West Australian adults who are likely to die within six months to legally access a lethal drug to end their life.

Australia's 'most dangerous legislation'

Mr Goiran said he still had serious concerns about the legislation.

"There's been 55 amendments passed, 25 from me and including very significant matters," he said.

"I'm sure most West Australians would agree that a doctor should not be a beneficiary under the will of a patient who's going to die, and that was allowed to stand all the way through the Lower House debate.

Nick Goiran says a number of concerning issues with the proposed laws have not been addressed. ( ABC News: Benjamin Gubana )

"There's a number of issues that have not been addressed which really concern me, not the least of which is that the bill still allows for self-administration completely unsupervised.

"This will be the most dangerous legislation that's ever passed Australia. It's more dangerous than the Victoria legislation, it's more dangerous than the [repealed] NT legislation."