Labor and the Greens are on the verge of a comprehensive preference deal that would boost the government's prospects of holding on to power while helping the Greens achieve the balance of power in the Senate.

Sources said the deal was close to fruition and more comprehensive than that crunched at the last election, while the Greens leader, Bob Brown, indicated it was all but done.

The agreement would give Labor crucial Greens preferences for House of Representatives seats in all but a handful of electorates where Greens branches will exercise their right to choose otherwise. In return, Labor will direct its Senate preferences towards the Greens.

As the campaign moved into full swing yesterday, the Prime Minister, Julia Gillard, declared the election a referendum on ''our very quality of life'' as she revealed a policy on sustainable population.