The NSW government is being accused of incompetence over a decision to exempt hybrid vehicles from an increased weight tax just days after the release of its transport blueprint.

The vehicle weight tax changes were announced as part of the blueprint, released on Sunday, with motorists slugged up to $30 a year more to help pay for the $51.2 billion plan.

The policy targets large cars over 975kg, but because it was based solely on weight, it also hit hybrid cars such as the Toyota Prius.

On Tuesday, Premier Kristina Keneally announced hybrids were to be exempted from the tax increase, a belated change seized on by the opposition.

Opposition Leader Barry O’Farrell accused the government of “making it up as it goes”, while opposition transport spokeswoman Gladys Berejiklian said Ms Keneally was guilty of “mistake after mistake”.

The opposition has described the tax change as unfair on regional motorists, who will get no benefit out of improved Sydney transport.

“She can’t tell us why she keeps making exemptions to the $30 tax on cars because they got that so wrong,” Ms Berejiklian told reporters on Wednesday.

“They make an announcement and then try to fix it up.”

Ms Keneally said the government decided to exempt hybrid cars after “listening to the community and responding”.

“I would have thought the opposition would have welcomed that hybrids were exempted from this scheme,” she said.

“We recognise that people who buy hybrid vehicles are already very mindful of carbon emissions.”

The transport blueprint again dominated question time, with Ms Berejiklian calling for Mr Campbell to resign over the hundreds of millions in taxpayer dollars spent on the scrapped CBD Metro.

“If this was the private sector the person would have been sacked long ago,” she said.

The premier was also grilled over the funding details of the blueprint, and proposals such as the $6.7 billion northwest rail link.

“It’s a $50 billion plan, fully funded, written into our budget, written into the state infrastructure strategy,” Ms Keneally repeated.

“It is the reallocation of the money we would have spent on the CBD Metro, and we are allocating money from the forward estimates.”

The stock response prompted an exasperated shadow Leader of the House, Andrew Piccoli, to his feet.

“The question was about detail. Just saying it is fully funded doesn’t mean it is fully funded,” he said