Pension cuts worth $2.4 billion are likely to face a tough passage through Parliament after the Federal Opposition announced it would not support the changes.

In the May budget the Government moved to target pensioners described as "liquid assets millionaires" by increasing the threshold for the assets test and the rate at which pensions reduce when a pensioner owns above the threshold.

But Labor has decided to oppose the measure, saying the wealthiest couples targeted by the measure receive only $55,000 in super income, even if their super savings are more than $1 million.

It cited modelling from Industry Super Australia that found the cuts would affect half of all workers due to retire in the next decade — more than a million people — including almost 330,000 part-pensioners who would feel the cuts immediately.

Key points: Eligibility for part pension tightened, with couples with assets in excess of $823,000 to miss out

Eligibility for part pension tightened, with couples with assets in excess of $823,000 to miss out Around 300,000 people will be affected

Around 300,000 people will be affected But maximum value of assets people can own and still get the full pension will be increased

Labor leader Bill Shorten addressed MPs at Tuesday morning's Caucus meeting, saying that the decision had been made because the pensioners affected were not rich people on high incomes.

He said 90,000 people would lose the pension entirely.

"We will stand by Australians who have worked hard all their lives and paid taxes all their lives and we will offer security to a new generation of Australians currently coming to the end of their working life, planning and saving for their retirement," Mr Shorten said.

Families spokeswoman Jenny Macklin said Labor wanted to protect people who are in the final years of preparing for retirement.

"People in their 50s and 60s who are contemplating and planning their retirement right now have to know that these cuts will hurt them," she said.

The Labor modelling showed a couple on the aged pension who owned their home and lived on a combined income of $45,000 would lose their entire part pension of $11,400 a year.

But Prime Minister Tony Abbott pointed to another change in the Government's pensions plan that would boost the maximum value of assets allowed on the full pension, from $202,000 to $250,000 for single home owners.

"Today, the Opposition decided to deny to 170,000 people with relatively low assets a pension increase of $30 a fortnight," he said in Question Time.

"They don't want poorer pensioners to get more money - what they do want is millionaire pensioners to keep their part pensions."

The Opposition said tax breaks for superannuation should instead be targeted, saying its policy to do so would save the budget $14 billion over a decade.

At the moment, couples can own $1.15 million in assets on top of their family home and still qualify for the part pension.

Under the budget cuts, the threshold would be reduced to $823,000.

According to Government figures released earlier, that would cut off about 91,000 from the pension and reduce it for another 235,000.

When the Government announced the measures last month, the Greens said they would consider them but indicated they wanted them to go before a Senate inquiry.

The changes are due to come into effect in January 2017.