Richard Di Natale has said that Bill Shorten will be the next prime minister and warned the Greens will use their mandate to pressure Labor “to be as ambitious as it possibly can be” on emissions reduction.

The Greens leader made the comments at the National Press Club on Wednesday in a speech proposing a government-owned electricity retailer in a bid to save households up to $200 a year on bills.

The tough rhetoric on climate change contrasts with a speech Di Natale gave earlier in September promising the Greens will “work with a new government to get climate action back on track”, an olive branch interpreted to mean it will not block Labor’s climate legislation.

With the Morrison government committing to the Paris agreement’s emissions reduction targets but with no mechanism to achieve them, Labor faces a policy test of its own in choosing whether to pursue its 45% renewable energy target through the Turnbull government’s national energy guarantee or through other means.

On Wednesday Di Natale said that Shorten “is going to be the next prime minister of this country – barring a catastrophe”.

The Greens leader said a Shorten-led Labor government would not pursue progressive policies like increasing Newstart, redirecting the private health insurance rebate to public healthcare and creating government-owned providers in electricity and aged care sectors.

Asked if the Greens would recognise a Labor mandate if they win the election, Di Natale replied that voters who “put a No 1 next to the Greens … are giving us a mandate”.

“They are saying to us – we want you to do everything you can to stop the Adani coalmine, we want you to make that transition to renewable energy a real one and a fast one.”

Di Natale repeated that the Greens “will work with a Shorten Labor government to get climate change back on track” but questioned if Labor wins a big majority whether it will “deliver the change that we need when it comes to climate change”.

“No, they won’t, and that’s why we need to have the Greens in the Senate.”

Asked if Labor attempted to legislate the national energy guarantee with a 45% emissions reduction target in the electricity sector – an increase on the 26% target proposed by Malcolm Turnbull before he was dumped as Liberal leader – Di Natale said he would not “prejudge what we will and won’t vote for”.

He said if Labor pursues a 45% target in the electricity sector “they’re walking away from their own 45% economy-wide emissions reduction target”.

“Because if you lock in 45% in the energy market, you’re saying we need to achieve a commensurate 45% reduction in transport, in agriculture, and no one believes that that is plausible.”

Di Natale said he feared Labor is going to implement the Coalition’s national energy guarantee with “weak targets” and “locking in this country into a trajectory which is not going to meet the targets we need to meet”.

Di Natale accused Labor of abandoning the emissions intensity scheme it promised at the last election, arguing he “can’t remember” the last time Labor had spruiked its benefits.

“I don’t trust the Labor party to deliver on climate change,” he said. “We know that the only reason we got a price on carbon and the only reason we got the Clean Energy Finance Corporation is because we had Greens in the balance of power in both houses.”

Di Natale suggested extending the renewable energy target beyond 2020.