More than two thirds of voters support legalising gay marriage in a Fairfax/Ipsos poll that shows Labor again ahead of the Coalition on a two-party basis.

The poll also found 57% believed that in the area where they lived it was not affordable for a first time buyer to purchase a home; 40% said it was. Bill Shorten has regained a narrow lead as preferred prime minister: 42%, up 3 points since May, to Tony Abbott’s 41%, down 3 points.

The opposition’s two-party margin is now 53-47%, compared with the Ipsos poll immediately after the budget, which saw government and Labor level. This was inconsistent with other polls at the time.

Labor’s primary vote is up 2 points since May to 37%; the Coalition has fallen 3 points to 40%.

Tony Abbott’s approval is down 2 points to 40%; his disapproval has risen by 4 points to 54%, for a net approval of minus 14.

Shorten’s approval is unchanged at 41%; his disapproval is up 2 points to 47%. His net approval is minus 6.

The Ipsos poll confirms other surveys in indicating same sex marriage has strong support – 68% are in favour and only 25% against. This is a rise of 3 points since the question was asked in a Fairfax poll in 2013.

However the issue is becoming increasingly politically complicated for Abbott, with some Liberals strongly backing a conscience vote, but an increasing backlash from conservatives in the party and in the Coalition’s base campaigning to keep its present policy of opposition to gay marriage and to prevent Liberal MPs being allowed a conscience vote.

Marriage equality is supported in the poll by a majority of Coalition voters (57%) and overwhelming majorities of ALP (74%) and Green (90%) voters.

There are differences between age groups but all are in favour: 18-24 (86%); 25-39 (75%); 40-54 (72%); 55 and over (53%).

Women are more likely than men to support marriage equality (74-63%), as are Australian born compared with foreign born (71-60%).

On housing, opinions about affordability for first buyers varies according to where people live. Among those living outside capital cities, 60% believe housing is affordable in their area, and 35% do not.

Only 29% of capital city residents think buying a first home is affordable – 69% do not. In Sydney just 18% say buying a first home in their area is affordable; an overwhelming 80% disagree. The poll of 1401 was taken Thursday to Saturday.

Michelle Grattan does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.