The two giant rubber ducks will be leaving Victoria Harbour a week earlier than planned after facing "unexpected challenges" during their time in Hong Kong, organisers said on Friday.

The pair went on show on June 10 and despite one temporarily succumbing to the summer heat, they were expected to remain off Tamar Park for around two weeks.

But they'll now depart on June 18 after one of them goes on a two-hour swim past Causeway Bay, Fortress Hill and Tsim Sha Tsui from 1pm.

"According to our observation and discussion with professional teams, it would be the best time to bring the exhibition to a successful close on Sunday, taking into account various factors such as operational costs," organisers said.

They thanked the crowds of enthusiastic people who have flocked to see the ducks.