Australian's 'brilliant career' ends; The End of My Career, by Miles Franklin. New York: St. Martin's Press. 234 pp. $ 10.95.

And then? And then?

Often, when we finish an enchanting book that made us love the characters and care what happens to them, we are left to wonder: What might have taken place if the story had gone on? We yearn for a sequel.

In the case of ''My Brilliant Career,'' Miles Franklin's turn-of-the-century novel about an irrepressible girl's emergence from adolescence in the harsh, wild, Australian Outback, we are fortunate. The author has given us a sequel, ''The End of My Career.''

It picks up the spunky heroine, Sybylla Penelope Melvyn, at a poignant and provocative moment at the end of the book ''My Brilliant Career.'' The scene is familiar to many who saw the film based on the book. Sybylla has just defied convention by refusing to marry, and, instead, sends off her first manuscript in a bid to become a writer and lead a life of independence.

Every dreamer's heart goes out with that bulky, brown-paper package, wanting to know: Did the book sell?

The sequel says yes, even as Miles Franklin's ''My Brilliant Career,'' written at age 16, sold when she was 21.

Did Sybylla's fame bring her happiness and good fortune? No. She received precious little money and all the unhappiness of a modest person thrust into the public limelight.

Sybylla's first shock was the literalness with which the imaginative ''yarn'' she concocted was received. The people of her community saw themselves as characters in 'Possum Gully. They were insulted to see their wretched poverty and narrow views so vividly described in print. It was assumed the story was autobiographical, and soon the townfolk and the clergy were castigating Miles Franklin's parents for raising so unorthodox and heretical a daughter.

For sensitive, innocent Sybylla, the anguish of discovering she had brought pain to her parents was unbearable. She fled to Sydney, where the literary lions proved as ruthless as the bush people she had left behind. She had many suitors, most of them unsuitable, and soon decided that being a person in her own right would always stand in the way of romance, however much she longed for a union of sweetness and equality.

Worst of all was the isolation of being a writer and a lone woman. She left Sydney but could not stay home in 'Possum Gully. She fled her beloved homeland and became an exile, a terrible price to pay for being an original thinker, for questioning conventional wisdom, for daring to be different.

All this is not to say the book is without joy. Sybylla was too rambunctious and full of girlish vitality to allow that. This book is at times a delicious satire on morals and manners. At other times it is a heart-rending tract for feminism. Always it is entertaining and filled with wisdom and universal truths.

Still, ''The End of My Career'' is not as carefully crafted as Franklin's first novel, perhaps because the innocence of the leading character and of Miles Franklin herself is gone.

It is difficult to separate Sybylla from Stella Maria Sarah Miles Franklin, who created her, because their lives paralleled each other in so many ways. Both were tomboys, accomplished horsewomen, and interested in the politics of their day, all of which was considered unladylike.

Miles Franklin (1879-1954) worked as a maid in Sydney while writing ''The End of My Career.'' Discouraged by its rejection as ''too audacious'' and ''outspoken,'' she moved to America and lived there for 30 years, writing and working for woman suffrage.

She returned to her beloved Australia, but never to the Outback. Perhaps more of her books, including the Bin Bin series, will re-emerge, in light of the contemporary interest in the freshness of her ideas and writings.

And, we must wonder, will there be another film?

And then? And then?