Big difference and anon - the FIBA rules require the team to have a nominated captain. The various roles of the captain are spelled out in the FIBA rule book. Among others, the captain is credited with points that might be scored by a defender accidentally tipping the ball through the other team's basket.
When the nominated captain leaves the floor and goes to the bench, the team has to nominate a player on the floor who fills the role of the captain. As such, there always has to be a captain on the floor, as stated by Anonymous #348480, at least in games run under FIBA rules
I don't know whether NBL retains a similar rule or a derivative thereof.
The FIBA rules can be checked online on the FIBA web site, if you're interested in the details.