Malcolm Turnbull’s popularity as preferred prime minister has slipped significantly as a difficult week in Parliament looms for the Australian leader and the Coalition.

Mr Turnbull’s lead over Labor leader Bill Shorten as preferred prime minister was slashed from 19 to 12 points in the latest Newspoll published in The Australian on Monday, as the ALP maintains its election winning two-party preferred lead..

Labor leads the government 51-49 on a two-party preferred basis. The major parties haven’t budged on a two-party basis for four Newspolls in a row since the beginning of July.

The latest poll, however, contains some additional bad news for the Prime Minister, with the coalition’s primary vote down two points to 37 per cent.

The ALP’s primary vote has fallen one percentage point to 35 per cent, the Newspoll suggests.

Federal politicians are returning to Canberra after the six-week winter break, with the government’s signature energy policy and proposed corporate tax cuts facing make-or-break decisions in the next fortnight.

The Prime Minister faces the task of selling the National Energy Guarantee to his own party room, after the states and territories offered their in-principle support.

The agreement is aimed at lowering power prices and restoring investment certainty after a decade of policy indecision.

Senators, meanwhile, will spend much of this week debating whether to restore the right of territories to legalise voluntary euthanasia.

Senator David Leyonhjelm wants Mr Turnbull to keep his alleged agreement to guarantee a vote in both houses on legislation to lift the ban on the ACT and Northern Territory controlling their own euthanasia laws.

The Liberal Democrats senator said staff witnessed the deal he says Prime Minister agreed to in return for his support for re-establishing the Australian Building and Construction Commission.

“He has a problem with his own party. There are conservatives within his party who basically, they hate the Prime Minister more than they hate the opposition for reasons best known to themselves,” Senator Leyonhjelm told the ABC on Sunday night.

“They are looking for every opportunity to give him a hard time. He is very sensitive to that. They are social conservatives so he did not want to give them any ammunition,” he said.

Senator Leyonhjelm said he wouldn’t give his support for upcoming legislation in the Senate, including on industrial relations and a tax on NBN customers, unless his deal is kept.

-with AAP