The old-age living allowance to be introduced in April is expected to cover Hongkongers in Guangdong by the end of next year.
Welfare secretary Matthew Cheung Kin-chung said the government would extend the means-tested HK$2,200-a-month allowance to Hongkongers who retired across the border after the scheme had been in place for more than a year.
The HK$1,090-a- month "fruit money" - means-tested for those aged 65 to 70 - is due to be extended to people living in Guangdong in September.
But welfare groups have warned that extending the allowance to those living outside the government's jurisdiction would be complicated.
"A lot of issues will need to be thought out first," said Chua Hoi-wai, business director of the Hong Kong Council of Social Service and a member of the Commission on Poverty.
"The government must think carefully about the details, feasibility and effectiveness of such a move," Chua said, questioning whether the move was the best solution or best use of resources.
"It's worth exploring, but no one should make hasty decisions. Guangdong is outside Hong Kong's jurisdiction, so it would be hard to apply the means test and track assets effectively. The elderly who choose to retire in Guangdong may also be in different situations to those in Hong Kong. The cash would help, but a more desperate need is for medical services."
Bella Luk Po-chu, executive director of elderly services group Helping Hand, which operates a care home in Zhaoqing , said the extra money would solve immediate needs.
She said providing the elderly with enough for a decent living on the mainland would take a burden off Hong Kong.
"The elderly would be able to spend their healthy years on the mainland," Luk said, admitting many would return when they needed medical attention.
Zero Kwok Wai-sang, senior manager of St James' Settlement elderly service centre, said he supported the idea of paying the allowance across the border but there should be a reassessment of asset limits and clearer rules and details for it to work.
St James' has seven centres for the elderly in Hong Kong, serving up to 7,000 people.
"The extra money could definitely raise elderly people's living standards," Kwok said. "But the government needs to clarify things. Do assets on the mainland count? With a lower living standard on the mainland, should the means test be readjusted? There are many kinks that need to be ironed out."
While the government had tried to simplify the allowance scheme, Luk said it was still quite complicated for many, especially with the means test.
The first group of 290,000 elderly already on record as passing the means tests will receive the HK$2,200 allowance on April 5, back-dated to December.