The Royal Bank of Scotland (RBS) will have to pay more than A$19m ($18m) to Oasis Fund Management and three other parties over the failure of A$65m ($62m) in principal-protected income notes, an Australian court has ruled.

The settlement was approved this week by New South Wales Supreme Court Justice, John Sackar, and brings to an end a four-year legal battle between ANZ-owned Oasis Fund Management and Oasis Asset Management versus ABN Amro Bank - which is now part of RBS in Australia - and Absolute Capital Investment managing director, Deon Joubert.

Oasis Fund Management and three other parties filed a suit in 2008 alleging that ABN Amro and Joubert had misled or deceived investors through the product's marketing materials and documentation.

According to the product's literature, funds were to be invested to generate interest at 8% while ABN Amro pledged to pay up to 25% of the investment's value if there was a shortfall.

ABN Amro had invested a quarter of deposits in cash which earned interest of under 5%, while Oasis and investors claim the product's information specified it would be invested in a portfolio of loans, financing facilities and other structured investments.

After the product failed in 2007, Absolute Capital returned A$17.4m ($16.9) while the balance left from ABN Amro's cash bets paid out another A$6.2m ($6m).

More than $62m was invested in the notes in 2003 and 2004, with 309 of the investors over the age of 60.

"In my opinion the settlement reached was indeed fair and reasonable," Sackar wrote in a 21 May judgment.

"Whilst the amount each will receive will involve accepting an amount less than that which they invested, it is nonetheless reasonable in all the circumstances."