Duet will invest $144 million in the Dampier-Bunbury pipeline (DBP) from the outset, while Alcoa will invest $36 million over about two years, a shorter time than envisaged when the investment was announced last month. The funds will be used to repay debt so the pipeline can retain its existing credit rating.

The phased schedule for Alcoa's investment results in Duet taking a larger share of distributions from the pipeline, beyond its 80 per cent equity interest, over those two years.

Duet's share of distributions will be 82.1 per cent, reverting back to 80 per cent gradually as Alcoa spends its share of the costs.

When Duet announced its $397 million entitlement offer last month, it said it would invest $160 million in Damper-Bunbury pipeline, while Alcoa would invest $40 million over 3-5 years. However the aluminium producer has since agreed to accelerate its investment, reducing the total spend to $180 million from $200 million.

"The invested funds will be applied by DBP to repay external senior debt and, in doing so, is expected to strengthen DBP's balance sheet and access to global debt capital markets," Duet said in a statement on Monday.

DBP is rated BBB- with a negative outlook by Standard & Poor's giving it the lowest investment grade. S&P said last month that a paydown of DBP's senior debt should be supportive to its financial metrics and that it would reassess the impact on completion of the recapitalisation.

The $16 million reduction in the amount Duet needs to pay into DBP will be retained by the company to fund other investment opportunities, it said.

Duet also allocated some funds from its capital raising to its Victorian gas and electricity distribution companies MultiNet Gas and United Energy.