It means Shell's 55-billion pounds (USD 84-billion, 79 billion-euros) takeover of BG remains on track for completion in early 2016.
"BG Group plc today confirms that the recommended cash and share offer for the company to be made by Royal Dutch Shell has received unconditional merger clearance from the Australian Competition and Consumer Commission," a BG statement said.
Separately, ACCC chairman Rod Sims said the "proposed acquisition would be unlikely to substantially lessen competition in the wholesale natural gas market, in either Queensland or eastern Australia more broadly".
The proposed transaction requires also support from shareholders of both companies.
The BG deal is aimed at helping Shell boost its flagging output thanks to BG's strong position in liquefied natural gas (LNG), a cleaner alternative to coal and nuclear energy.