Act whip Brooke van Velden said the party was "offered two places on the upcoming tours but we have declined the invitation".
"These tours are expensive and it's taxpayers who pick up the bill. No clear purpose that serves the public interest has been presented, leaving only the MPs to benefit. It would be especially inappropriate to go on a taxpayer-funded junket without a clear policy purpose when debt and inflation are at record highs and Kiwis are feeling the pinch," she said.
But Mallard said the trip was worthwhile.
He said that once the European Union Free Trade Agreement was settled, it would need to be ratified by local parliaments, so it was important for New Zealand's parliamentarians to have good relationships with their European counterparts.
He said the MPs would be "doing some work" to position New Zealand for the deal.
Mallard said there would be "meetings with people involved in primary production", and that he had requested the trip include members with practical farming experience.
Alongside Collins, National's Agriculture spokeswoman Barbara Kuriger will travel on the trip.
Labour will be represented by Duncan Webb and Helen White.
"It is the first delegation since 2019, and there was a lot of discussion about Europe being the priority because of the FTA," Mallard said.
"It is the biggest thing that parliamentarians will be involved in that affects New Zealand probably for the next five years," he said.
"It is in the interest of every New Zealander that we have a successful free trade agreement," Mallard said.
The delegations will meet Parliamentarians on each stop, and will also meet members of the European Parliament.
This is particularly important in Belgium, when MPs will meet with some regional parliamentarians. Belgium's powerful regional parliamentarians have a history of slowing trade deals.
Mallard will also meet with other Speakers, or Speaker-equivalents on the trip, including the President of the European Parliament.
The Ukrainian conflict will loom over the trip. MPs will have a security briefing in Rome, and will meet people associated with efforts to help refugees in Poland.