Victorian MPs have sat through the night in Parliament in a heated debate over the Government's assisted dying legislation which is still continuing this morning.

The debate on the controversial laws began at 9:30am in the Lower House on Thursday, running all day and overnight.

It is still underway and it is unclear when the final vote will take place.

The legislation had also been debated on Tuesday and Wednesday.

Tempers began to flare last night as those opposed to the bill tried but failed to pass more than a dozen clauses and hundreds of amendments.

The amendments called for changes like publicly disclosing euthanasia statistics and prohibiting advertising for voluntary assisted dying.

Liberal MP Graham Watt said Victoria would become a "suicide tourism destination" if the laws were passed.

His colleague Tim Smith, who also opposes the legislation, said Parliament should be adjourned.

"The fact that we've sat now literally all night has just been ridiculous," he said.

MPs have been debating the legislation all week. ( AAP: David Crosling )

The new laws propose that people over the age of 18 with a terminal illness with no more than 12 months to live and who are in severe pain, will be able to request from doctors a lethal medication.

Another proposed amendment would have changed the criteria to limit access to assisted dying to those with six months to live.

Politicians on both sides have been escaping to their offices to nap in between votes on various amendments.

The cafeteria has stayed open all night, serving coffee to the growing ranks of the weary.

But the past 24 hours have perhaps been most gruelling for Health Minister Jill Hennessy and Attorney-General Martin Pakula, who have needed to be available to answer questions on the bill.

The laws would allow medical practitioners to decide whether the person's suffering can or cannot be relieved.

When asked to explain the definition of "suffering", Ms Hennessy said decisions would be made on a case-by-case basis.

"At ten past eight, we have hit the crossroads of existentialism, I think, in the course of this debate," she said.

"I am not optimistic I am going to be able to persuade you because I accept some of these [questions] represent deep philosophical differences.

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"It is qualified with that provision that that suffering cannot be relieved in a manner deemed tolerable to that patient."

Yesterday, Deputy Premier James Merlino failed in his attempt to block the bill, putting him at odds with Premier Daniel Andrews, who strongly supports voluntary assisted dying.

Liberal Box Hill MP Robert Clarke rallied against the bill, arguing supporters had been stubborn in their approach.

"It is obvious that the Government is determined to press on as it is and is not prepared to accept amendments, and unfortunately that is reinforcing the conclusion that this has been a one-eyed process."

Greens MP Ellen Sandell tweeted she was disappointed in the tactics to drag out the process and delay the vote.

"Filibustering on assisted dying legislation by Libs and Nats overnight has been really disappointing. Some appalling behaviour," she said.