Previously unseen footage of ABBA rehearsing Mamma Mia for their Bandstand appearance was broadcast in April 2004 as part of Channel 9's coverage of ABBA's 30th Anniversary and the fifth anniversary of the stage show "Mamma Mia!".

The footage was apparently originally rough snippets but has been edited together so the piece flows smoothly.

A smaller excerpt of the same footage was shown during a report on Channel 9 about ABBA Voyage and ABBA's ‘return’.

Bandstand Rehearsals (Mamma Mia), 1976;
Bandstand (Mamma Mia), 1976;
Dancing Queen (Bandstand), 1976;
The Best of ABBA (Bandstand Special) (Mamma Mia, Hasta Manana, Ring Ring, Tropical Loveland, Waterloo, I Do, I Do, I Do, I Do, I Do, Rock Me, (Lucky Starr), Honey, Honey, Fernando, So Long, SOS), 1976; 
ABBA in Australia (Mamma Mia, Hasta Manana, Ring Ring, Tropical Loveland, Waterloo, I Do, I Do, I Do, I Do, I Do, Rock Me, Dancing Queen, Honey Honey, Fernando, So Long, SOS, Bang-A-Boomerang), 1976;

Thanks to Ian Cole, Philip Muytjens and Harold Hanlon. 
YouTube link c/o bill patterson