Andrew Witty, chief executive of Britain's biggest drug maker, said last month during quarterly results that he was looking for further ways to simplify the GSK business, following previous divestments in consumer health.
The old Australian brands being bought by Aspen include herpes treatment Valtrex, epilepsy drug Lamictal and the antibiotic Amoxil. In total, the 25 products generated sales of around £83m in 2011 and £31m in the first half of 2012.
Revenues for these products have gradually declined over recent years due to generic competition.
The sale to Aspen is expected to complete in the fourth quarter, subject to regulatory approvals from Australian authorities, and GSK said net cash proceeds would be some £155m.
Aspen said in a statement that the purchase price for the 25 pharmaceutical products was subject to a small reduction should the deal not be completed by 31 October. It will fund the purchase from new offshore debt facilities.