The Greens have been refused permission by the ABC to use a fake advert aired on The Gruen Nation on Wednesday night.

The ad was prepared by Sydney agency Republic of Everyone for a segment called 'The Pitch'.

It featured the tag line: "If you think, vote The Greens."

Greens spokeswoman Ebony Bennett confirmed The Greens contacted the ABC and Republic of Everyone over the possibility of using the ad, but the ABC would not allow the clip to be used during the election.

The ABC says it cannot be seen to pick sides in the campaign.

An ABC spokesman was quoted as saying the advert was created solely for use on The Gruen Nation.

The Gruen Nation also provided reasons for the rejection on social networking site Twitter.

"@empatt Yep. All pitches in Gruen Nation and Transfer sign over copyright. Otherwise too risky for a organisation like the ABC to do it," it said on its Twitter account.

Greens leader Senator Bob Brown had earlier told reporters he thought the ad was great.

"The Greens have normally had a struggle to stay in election campaigns because of the big spending of the old parties," he said.

"We'll give them a run for their money this time, but the Gruen Nation ad was spot on."

Senator Brown was then asked if the Greens would approach the ad agency and ask if they could use it.

"A little birdie tells me that might have already happened," he said.