A number of other Labour MPs were not there either.
A spokesman for Ardern said it was normal to have MPs absent from a caucus meeting, especially if it had been called at short notice like this one.
He said Twyford was granted leave to go to the Planning Institute event in May.
The absent MPs and ministers would have "no impact" on the reshuffle decisions, the spokesman said.
Speaking at her post-Cabinet press conference on Monday, Ardern said the reshuffle would be "minor".
It will be her first reshuffle since she became Prime Minister and appointed her Cabinet in 2017.
The reshuffle was originally set to take place early this year, before being bumped back to after the Budget.
Ardern wouldn't say who she had spoken to, provide details of the process taken, or hint at who might be moving up or down.
"Ultimately they're my decisions," she said.
With the Government's flagship KiwiBuild programme struggling, questions have been raised about whether Transport and Housing Minister Phil Twyford may be in danger of demotion.
Pressed on whether she still had confidence in Twyford on Monday, Ardern leapt to his defence.
"He's done an incredible job with a very difficult area of work ... No government has had to do this before or tried to do this before," she said.
When questioned by media later in the week about the reshuffle, Twyford was giving nothing away.
He said it was for the Prime Minister to announce details of the reshuffle, not him.
