National's Judith Collins says someone told her about the phone call and she decided to check out whether it was right with a question in Parliament.
The Transport Minister says he apologises unreservedly over the matter.
It comes a day Ms Bennett walked out of the Chamber over a disagreement with the Speaker over supplementary questions.
Gerry Brownlee had expressed concerns about the way Mr Mallard was running question time.
The PM is speaking at Green Bay High where she launched a careers initiative today.
The suspects have all denied it. The trail of clues has gone cold. The opposition says there's no evidence it even happened.
A Maori health leader and anti-smoking campaigner warned politicians more needs to be done, and soon, to save lives.
National has sent a letter to Trevor Mallard accusing him of lacking neutrality towards the Government during Question Time in Parliament.
The National Party deputy got fed-up with the Speaker and decided to leave the House rather than continue to debate.
Jacinda Ardern said the $100m "was not actually allocated to any specific project",
The demands are to replace the deal set up by President Obama, which has been scuttled by President Trump.