The country's largest power generator - Meridian Energy - is reporting a sharp rise in profit, but warning tough months are ahead

Its underlying net profit is up 14-percent.

The partially state-owned electricity gen-tailer says it's investing almost as much into developing new and existing assets, as it's making in profit.

It says the operating environment changed dramatically in May when lake levels plummeted, which will affect its bottom line in coming months.

The company says a very small number of electricity users directly exposed to the wholesale market have been significantly impacted.