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The four-and-a-half storey billboard is part of a campaign by Powershop, with the slogan "Same Power, Different Attitude".
The signs have recently been put up in central Auckland and Wellington.
Powershop chief executive Ari Sargent said it had received a mostly positive reaction from the public.
The billboard's message of freedom of choice and equality aligned with the company's values, he said.
He said the billboard was not targeted at Catholics "per se", but the Pope was an analogy of big power companies.
"(It's) making the point that some larger institutions can often lose touch with their constituents."
It was never their intention to offend anyone, he said.
"We're certainly trying to provoke debate, we make no apology for that."
The Catholic Church in New Zealand had no immediate comment on the billboard.
Labour MP Louisa Wall's Private Member's Marriage [Amendment] Bill has passed its first reading and submissions are being heard by a select committee.
Powershop is owned by state-owned Meridian and lobby group Right to Life said it was inappropriate for it to be involved in politics and taking a position on the Bill.
Right to Life spokesman Ken Orr said Minister of State Owned Enterprises Tony Ryall should instruct Meridian Energy to have the "highly offensive" billboard removed immediately.
But Mr Sargent said it was "a bit of a stretch" to say that a few public billboards were going to influence public opinion.
"People will make their own choices if they are pro marriage equality or not."
Mr Orr also said it was insulting to Catholics and others in the community to "lampoon" the Pope.
He said the Pope was the respected religious leader of the Catholic Church which upheld the institution of marriage as being exclusively between one woman and one man.
Rainbow Youth organiser Tom Hamilton said anything around diversity was positive.
"If we've got a power company engaged in the diversity around the marriage topic, then good on them."
He didn't believe the company was using the sign as a marriage equality initiative, "it's just a little bit of something that's left of field".
The billboard received mixed messages from people passing by. One woman said it was "a bit stupid".
"I wouldn't have noticed it was about power." She said it had the potential to be controversial.
Clare Lenihan thought the billboard was amusing. "It's making fun of intolerant people. I think bigots might be offended by it."
A man who described himself as a Kiwi-Asian Baptist said it was "bad advertising" with potential religious implications.
But another passerby, Ben McKenzie, said the billboard was "pretty cool".
Powershop has courted controversy with other billboards, such one as depicting former Iraq president Saddam Hussein collecting charity and former North Korea dictator Kim Jong-il selling hotdogs for charity.
In a blog on its website, it said the "Same Power, Different Attitude" campaign took a "bunch of rotten demagogues, famous the world over for their abuse of power, and recasts them as people who do decent things in their community".
"It's satire for sure, but you could say we've got a bit of nerve to feature people in our ads who've regularly violated human rights. In truth, we think that dressing them up as humble, caring people is just about the best way possible to humiliate them."
The company did not mention the Pope billboard.