UPDATE 1.27PM: Statistics Minister James Shaw has rejected National's "missing in action" criticism.

The opposition earlier on Tuesday accused him of "swanning around the Pacific" while his department grapples with census problems.

Former statistics minister Scott Simpson said it was "unbelievable" Mr Shaw was out the country during the most important interaction his department has with the public.

"He has chosen to be swanning around the Pacific on a junket while his officials at Statistics NZ are left to carry the can," he said.

Shaw is with Prime Minister Jacinda Ardern on her Pacific Mission, in his capacity as climate change minister.

"Climate change is a top priority for the islands," he told NZ Newswire.

"National largely ignored it, and it's important for me to be here as part of the mission."

Shaw said National was well aware that there was "a pretty clear operational wall" between Statistics NZ and the minister.

When Simpson said Shaw should be at home, he cited concerns about people who not received their census code number letters, older people struggling with online responses and the lack of response to queries and calls for help.

The Statistics Department has given assurances it is on top of the issues around the census.

It says letters will soon be on the way to households which haven't received their online access number, and people who ask for paper forms will receive them.

Although Tuesday is Census Day, the forms can be completed before or after it so long as they show the state of households on March 6.

The department's general manager, Denise McGregor, says codes have been delivered to 96 per cent of the country.

After Tuesday, field teams will be out following up on households that haven't responded.

By midnight Monday 1.74 million forms had been filled out online, a figure the department says is "promising".