Ms Morton, the author of five bestsellers including The Shifting Fog and The Lake House, has earned more than $17.3 million in royalties from her novels since she broke onto the literary scene in 2005.

But Ms Morton claims her long-time literary agent Selwa​ Anthony deprived her of further income by failing to get her the best publishing deal when she was an unpublished and inexperienced author.

Ms Anthony, who represents some of the biggest names in Australian popular fiction including Tara Moss, Belinda Alexander and Diane Armstrong, took legal action in the NSW Supreme Court after Ms Morton suddenly sacked her in December 2015 following a relationship spanning more than 10 years.

Ms Morton, who now lives with her family in Britain, had hired a new London-based agent to not only manage her future works, but to manage the royalty flow from her backlist.

Ms Anthony is suing Ms Morton for breach of contract, claiming the pair agreed during a telephone conversation in 2002 that the agent would get 15 per cent commission of all monies earned by the author for the life of the works for which she negotiated the publishing agreement.

In a statement of claim Ms Anthony claims this arrangement survives Ms Morton leaving her agency and joining another.

Ms Morton has earned $17.3 million in royalties from publishers for her novels, audio books and large print editions, of which Ms Anthony has taken $2.8 million commission. Ms Anthony has asked the court to order Ms Morton pay her 15 per cent of all royalties earned since January 2016 and for the life of each work.

But in a cross-claim, Ms Morton says Ms Anthony breached her fiduciary duty and duty of care by failing to secure the "most advantageous offers reasonably available in the circumstances" when she signed up with publishing giant Allen & Unwin.

Court documents reveal in 2004 Ms Morton, then an unpublished author, sent a partially completed manuscript of The Shifting Fog, to Ms Anthony. In May 2005 Ms Anthony forwarded an offer from Allen & Unwin of an $8000 advance, of which she kept $1200.

The debut novel, also known as The House at Riverton, was a Sunday Times #1 bestseller in Britain and a New York Times bestseller in 2008. It was named General Fiction Book of the Year at the 2007 Australian Book Industry Awards.

Ms Morton has enjoyed local and international success and critical acclaim, selling more than 10 million books in 42 countries and in 34 languages.

But she said that in 2004 Ms Anthony should have known that she was "an unpublished and inexperienced author, with no experience in business, who relied on Ms Anthony's expertise".

Ms Morton claims Ms Anthony favoured her own interests, had undisclosed conflicts of interest and "misused her position for personal gain".

In particular, Ms Morton claimed Ms Anthony failed to improve her position with Allen & Unwin "commensurate with her increasing commercial success", failed to communicate offers from other publishers, failed to provide proper accounting, reporting and sales information and deprived her of substantial revenue by transferring her rights on "uncommercial terms" including granting rights for the whole of the copyright period and granting future rights to "valuable unwritten books in advance".

Ms Morton claims Ms Anthony "failed to do what she was employed to do" and "should forfeit the remuneration she has been paid by Ms Morton".

In her defence, Ms Anthony denies all the claims and says Ms Morton had sent her two unpublished manuscripts before The Shifting Fog. She says she determined Allen & Unwin "was the most appropriate publisher to approach for a work of popular fiction by unpublished Australian author". She defended her practices as custom within the international book publishing industry. Further, she says all money received from publishers were promptly paid.

She also said Ms Morton was outside the limitation period to make some claims.

Ms Anthony declined to comment while the matter is before the court. Ms Morton's lawyer said the author was travelling and could not be contacted.

The case returns to court this month.