Tony Abbott faces an uphill battle with his budget sales pitch as the latest Newspoll shows the Coalition’s primary vote remains stable at 36%.

The poll, published in the Australian newspaper on Tuesday, indicates the past fortnight of political debate has failed to significantly shift voter sentiment.

The Liberal-National Coalition’s primary vote was unchanged at a four-year-low of 36%, Labor’s support fell one point to 37% and the Greens’ support rose one point to 12%. Based on preference flows at the last election, these figures translate to a two-party-preferred lead for Labor of 54% to 46%.

“We’ve got a big, big budget to market and to legislate,” the prime minister told the ABC on Tuesday. “We’ve got a very, very big job ahead of us. If I may say so … we may not be the most popular government at the moment but we are the hope of our side. If our country is to face the future with strength and confidence this government must succeed.”

In the past fortnight the government has sought to make the case for tough measures to tackle the “debt and deficit disaster”, but several ministers have confused details of their policies or appeared to support ideas later ruled out by the prime minister.

Abbott played down the mistakes: “Peter Costello once pointed out that every day was an exam and if you got 30 out of 30 nobody could give a rats, so to speak … 29 out of 30 was a pretty good mark, frankly, when I was at university.”

The opposition leader, Bill Shorten, has repeatedly accused the government of breaking promises and has signaled his determination to block key measures, including the $7 GP fee and the deregulation of university fees.

“Australians are giving Tony Abbott's budget the thumbs down,” Shorten said on Monday. “We now see, in the last 24 hours, a prime minister who knows that his budget is sinking fast, who can't control what his ministers say, and in the space of a 24-hour period we are seeing the government change its mind on key issues.”

The prime minister's personal approval rating has improved slightly since the immediate post-budget Newspoll. Approval of Abbott rose three points to 33% and disapproval declined one point to 59% – translating to a net approval rating of minus 26%.

Approval of the opposition leader, Bill Shorten, declined four points to 38% and disapproval rose four points to 43%, or a net approval score of minus 5%. Shorten led Abbott as preferred prime minister, 45% to 35%.

The Newspoll of 1,158 voters, taken from Friday to Sunday, has a maximum sampling error of three points.