Kem Ley, a popular analyst and critic who dabbled in grassroots political organising, was shot at point-blank range while having his morning coffee at a gas station cafe in Phnom Penh in July 2016.
"The Australian government granted them visas on Wednesday 14 February 2018 and they arrived in Australia on Saturday 17 February," the office of Victorian state MP Hong Lim, who helped with the application, told AFP.
The family was granted special humanitarian visas for those "subject to substantial discrimination amounting to a gross violation of your human rights in your home country".
They can apply for Australian citizenship after four years.
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Kem Ley criticised Cambodian politicians of all shades but he was particularly scathing about the corruption that blights the country.
Shortly before his murder he gave an interview about an investigative report that detailed some of the millions of dollars allegedly amassed by the family of long-serving Prime Minister Hun Sen.
Unemployed former soldier Oeuth Ang admitted carrying it out and was sentenced to life imprisonment in March after a brief trial.
But many observers and members of the public were sceptical of the story, which gave the motive as an unpaid USD 3,000 debt.
Kem Ley's murder shocked Cambodia's community of activists and civil society, and tens of thousands attended funeral ceremonies.
Political assassinations were common in the 1990s and early 2000s in Cambodia but have become rarer in recent years, with the legal system replacing violence as the chosen form of repression.