Outsource giant Serco said it remains on track to deliver on its 2009 guidance and continues to see significant opportunities in the current challenging economic environment.The group also announced that it has signed an A$370m (£180m) contract with the Australian Government's Department of Immigration and Citizenship to transform its immigration detention centres."We have seen a sustained high level of activity throughout the first half, trading remains strong, and we continue to generate good cash flow," said the group."Our broad capabilities and the flexibility we have in deploying them, together with our pipeline of opportunities in existing and new markets and our substantial order book, support our expectation of continued strong performance," it added.Serco expects a modest increase in working capital investment compared to the same period last year driven by a number of contract start ups and transitions, and acquisition integration.The group will release its 2009 half year results on 26 August.