THE Greens are widely tipped to hold the balance of power in the Senate after August 21 and have promised to use their numbers to push for tougher action on climate change and a more compassionate approach to asylum seekers.

Polling suggests the Greens could pick up an extra four Senate spots, taking their numbers from five to nine.

The Greens' Senate hopeful in NSW, Lee Rhiannon, is rated a 50-50 chance by pollsters of claiming a seat. The party needs 14.3 per cent of the vote to claim a quota.

It has been difficult for minor parties to achieve Senate success in NSW - the last being Kerry Nettle for the Greens in 2001. She lost her seat in 2007.

Ms Rhiannon will tomorrow resign her seat in the NSW Parliament. She had been under pressure to quit amid accusations she had used public funds to promote her federal campaign.

''The top priority is climate change. The massive coal developments happening in NSW are making this situation worse and we have to begin to turn away from coal-fired power and increasing coal exports,'' Ms Rhiannon said yesterday.

''The second would be overhauling election funding. We need to bring more democracy to this country.''

Cate Faehrmann will replace Ms Rhiannon in the NSW Parliament.

The election will be a half Senate election, which means only 38 of the 76 senators are up for re-election.

Among those are Family First Senator Steve Fielding who is tipped not to be re-elected. He was only successful in 2004 because Labor in Victoria chose to direct its preferences to him to stop the Greens from winning another Senate spot.

''Family First is the only party which is looking out for families when it comes to housing affordability, childcare and making our streets safe again,'' he said.

''People care about the rising cost of living and so far we've heard nothing about how Gillard or Abbott is going to ease our housing affordability crisis. It's time we took housing affordability seriously with 70 per cent of Generation Y of the belief that they are permanently locked out of the housing market.''

The Greens are hopeful of picking up the lower house seat of Melbourne, which Labor will fight to retain following the retirement of Finance Minister Lindsay Tanner.