(Adds details of offer, Tilt review) Dec 9 (Reuters) - Infratil...

(Adds details of offer, Tilt review)

Dec 9 (Reuters) - Infratil Ltd (IFT) on Wednesday rejected a NZ$5.37 billion ($3.78 billion) takeover offer from AustralianSuper, the largest pension fund in Australia, saying it undervalued the New Zealand-based energy and transport infrastructure investor.

Infratil said it would not engage further with AustralianSuper, even though it had recently sweetened its cash plus stock offer to NZ$7.43 per share, which was a 22.2% premium to Infratil's closing price on Tuesday.

In October, when the A$190-billion pension giant first approached Infratil, it had offered NZ$6.40 per share.

"Both proposals were unsolicited and materially undervalue our significant renewable energy and digital infrastructure platforms," Infratil Chief Executive Officer Marko Bogoievski said.

The decision comes after Infratil on Monday said it was looking into selling its 65.5% stake in wind farm firm Tilt Renewables (TLT) , which was worth around NZ$1.18 billion at Tuesday's close.

The AustralianSuper proposal offered each Infratil shareholder NZ$5.79 in cash and 0.2210 shares of Trustpower Ltd (TPW) , in which Infratil is the largest shareholder.

($1 = 1.4196 New Zealand dollars)