Ballot boxes containing early votes are allowed to be be taken home by polling booth staff if they can't be locked up safely at the venue overnight, the Electoral Commission says.

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But it would not say if this had happened to any advanced votes this election, citing security reasons.

"All ballot boxes and materials are regularly returned to electorate headquarters as part of our security processes. The only times ballot boxes will be secured at the home of an electoral official are when the venue has no secure lockable rooms or that the venue has multiple keyholders," an Electoral Commission spokeswoman said.

Advanced polling places opened on 11 September and figures from the Electoral Commission showed nearly 551,000 people had since cast their vote.

Asked how many times ballot boxes had been taken home by staff overnight since polling opened the Commission said: "For security reasons, we wouldn't go into any further detail."

Wellington lawyer Graeme Edgeler said he has no concerns about the practice as there were other security measures in place.

"If that's the safest place for them, then it's better than leaving them unsafe.

"These boxes have seals on them, so that if a box has been opened, the seal will have had to have been cut, and the new seal would be put in place, but that new seal has a different number on it."

The security of voting materials was taken very seriously and there were several measures in place to ensure they were secure, the Commission said.

"During the initial assessment of all of our advance voting places Returning Officers are required to assess storage options. We generally require access to a solid, lockable room or cupboard to which no one else will have access during the voting period.

"This, however, needs to be balanced with our ability to offer services in some public spaces. In instances where there are no suitable storage facilities on site, electoral officials/advance voting place managers may take materials back to the electorate headquarters. Only if none of these options is possible would we allow an electoral official to secure the voting materials at their home overnight."

All ballot boxes were sealed throughout the voting period, and additional protections included detailed tracking of all used and unused ballot papers including allocating ballot paper pad numbers to specific individuals, tracking of single-use security seals to individuals, a place on each ballot box to record the seal number in use at any time, daily reconciliation and reporting of ballot papers used for each electorate on site, the Electoral Commission said.