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Cheung Kong Infrastructure (CKI) has made an unsolicited offer to acquire Australian energy firm Duet Group for $5.4bn (£4.2bn).

CKI is a Hong Kong-based conglomerate, owned by billionaire Li Ka-shing.

The offer represents a 28% premium on Duet's share price, when last traded on Friday.

Any formal bid by CKI would be subject to approval from Australia's Foreign Investment Board Review (FIRB).

Earlier this year the Australian government rejected a $7.4bn combined bid by CKI and China's State Grid Corp for a controlling stake of state-owned energy grid, Ausgrid, citing security concerns.

In a statement to the Australian Securities Exchange, Duet said its board was considering the proposal, and it was also advising shareholders to "take no action as there was no certainty the proposal will proceed further".

CKI sees itself as a global infrastructure company with investments and operations in many countries including the Netherlands, Portugal, New Zealand and Canada.

Forbes magazine estimates that Mr Li's wealth from his property, energy, retail, ports and technology businesses amounts to $33.5bn (£26.2bn).