The A$1.1bn deal includes Oamps businesses in the UK
Gallagher has completed its purchase of Wesfarmers broking business after receiving regulatory approval.
The deal, first announced on 7 April, was valued at A$1.1bn (£608m).
The acquisition includes Oamps businesses in the UK and Australia, as well as Crombie Lockwood in New Zealand and its associated premium funding companies.
Arthur J Gallagher president and chief executive Patrick Gallagher has previously said following the acquisition, the combined operations will become one of the largest brokers in Australia and New Zealand.