The series is a 1995 co-production between Australia and Poland, is one single complete story given in a serial form — in the same way that a complete story is sometimes given in a television mini-series. The series has 26 episodes altogether, with all of the episodes continuing immediately on from the previous episode in a serialised fashion, including having a cliffhanger at the end of each episode.
An interesting note is that many of the people from Riana's world speak in an Eastern European accent, with their names sounding like names from the same geographical area. This is because most of the scenes from the alternate universe were in fact filmed on location in Poland and featured Polish actors.
P.S. It was Nickelodeon