UPDATE 4-Canada opposition against Nexen deal, fears tidal wave of bids

* Canada NDP says it fears "tidal wave" of foreign bids

* Government deciding whether to approve CNOOC bid for Nexen

* NDP cites lack of transparency in approval process

* Nexen shares lose one percent after comments

(Adds comments by opposition) By David Ljunggren

OTTAWA, Oct 4 (Reuters) - Canada's main opposition party on Thursday demanded the government reject a Chinese state-owned company's landmark $15.1 billion bid for oil producer Nexen Inc , saying approval of the deal could trigger "a tidal wave" of foreign takeovers.

The comments by the center-left New Democrats, or NDP, highlighted the risks faced by CNOOC Ltd's bid, China's largest-ever proposed foreign takeover, and helped knock almost 1 percent off Nexen shares.

The NDP has no power to block the deal, but Conservative Prime Minister Stephen Harper says Ottawa will take public opinion into account when deciding whether the proposed takeover will have a net benefit of Canada.

Some legislators in the governing Conservative Party are also uneasy about the idea of a Chinese state-owned enterprise buying up more Canadian oil assets. Canada, a leading energy exporter, has the world's third-largest proven oil reserves.

NDP natural resources spokesman Peter Julian complained the criteria for determining "net benefit" was far too vague and excluded questions about jobs, human rights, national security and the environment.

"Public confidence in the government's ability to actually handle this transaction fairly, to do it in a transparent way, has eroded," he told a news conference.

"The New Democratic Party cannot support the rubber stamping of the CNOOC takeover of Nexen. We cannot see the net benefit."

Nexen shares, which began to drop after Julian started speaking at 10 a.m. (1400 GMT), were trading at C$24.99, down from C$25.17. They later slipped to C$24.96.

Fund managers and analysts still expect Ottawa to approve the deal, albeit with conditions.

Julian, who wants the government to hold public hearings on the bid, cited what he said was the risk of "a number of other takeover deals that are pending. Some people have said it's a tidal wave of takeovers that are coming down the pike."

Industry Minister Christian Paradis, ultimately responsible for deciding whether to approve the CNOOC bid, said the NDP's actions were reckless and irresponsible.

"By attempting to politicize the review process they are creating the kind of uncertainty that scares off the investment Canadian companies rely on to create jobs, innovate and compete," he said in a statement.

The government says the energy patch needs C$630 billion ($643 billion) in investment over the next decade alone and much of it will have to come from outside Canada.

The NDP said it could not support the deal as it was currently structured and demanded Ottawa be much more specific about what it expected from foreign investors.

"It's absolutely absurd that what we have is an industry minister with nebulous criteria, basically drawing up a response on a napkin," said Julian.

Although Julian did not say exactly what changes he wanted made to the CNOOC bid, he said the government should be able to force companies to live up to promises -- such as maintaining jobs -- they made when making a takeover bid.

The New Democrats, Canada's most left-wing mainstream party, unexpectedly became the official opposition for the first time last year.

The NDP said its stance on Nexen was based both on the deal itself and on broader concerns about the pace of development in northern Alberta's oil sands, one of the world's biggest crude oil deposits, where Nexen has a small stake.

"We have to be strategic. The resources are going to be there," NDP industry spokeswoman Helene LeBlanc told reporters. "They're not going away, and I think we can be strategic...(on) the rate of development of the natural resource."

The Conservative-dominated House of Commons rejected an NDP motion on Wednesday that demanded public consultations on the CNOOC deal. The vote was 145 against the motion to 125 for it.

($1=$0.98 Canadian)

(Additional reporting by Randall Palmer and Louise Egan; Editing by Janet Guttsman, Frank McGurty)

((david.ljunggren@thomsonreuters.com)(+1 613 235 6745)(fax +1 613 235 5890)(Reuters Messaging: david.ljunggren@thomsonreuters.com))