The government on Monday sought to alleviate privacy concerns over the potential future use of smart ballot boxes that can detect invalid votes.

Constitutional affairs minister Erick Tsang told lawmakers that officials are testing out the devices designed to help voters fill out ballot papers properly.

The DAB's Chan Wing-kwong expressed concern over how votes will remain confidential if the ballots are analysed by the smart boxes at a Legco constitutional affairs panel meeting.

Election Committee lawmaker Tang Fei said voters might feel embarrassed if they are informed they have made a mistake.

Third Side lawmaker Tik Chi-yuen said ballot-checking might amount to election interference.

Tsang said the idea of bringing in the smart boxes is to protect voters' rights in the event that they inadvertently fill out ballots incorrectly, for example if a voter fails to tick the 40 boxes required for Legco's Election Committee constituency.

"We are not trying to identify voters who have deliberately defaced the votes. When people are actually casting their ballot, there is no CCTV involved, and you don't have to produce your ID card. And there is absolutely no way for us to identify who you are," the minister said.

"So the privacy of voters is preserved. Our main purpose really is to reduce the situation where erroneously, voters tick the boxes leading to the vote becoming invalid."

Tsang said similar technology worked well in two recent elections.

The smart ballot box is included in proposed legal changes the government plans to table to the legislature by the end of March.

Officials have not decided in which elections the boxes can be used, with Tsang saying they might not be used in Legco polls next year.