The offer price at US$ 2.18 per share represents a 25 % premium to Centrebet's Tuesday close.
Sportingbet said it expected the acquisition to add to its earnings in the first full year post integration. "An acquisition would accelerate Sportingbet's strategy of increasing its exposure to regulated markets and of geographic diversification," CEO Andrew McIver said in a statement.
Centrebet, which offers up to 6,000 international sports and horse racing markets per week, reported a first-half net profit of US$ 1.96 million, down 70.6 % from last year.
Sportingbet shares were up 3.3 % at 47.5 pence at 0705 GMT on the London Stock Exchange, while those of Centrebet closed up 12.6 % at US$ 1.95 on Wednesday on the Australian Stock Exchange.