Check out the following article discussing recent EKOS poll results.  Here's what the pollster has to say:

"There is a shock result here for the NDP," Graves said. "Nationally, the Greens are within sight of the NDP."

The NDP is at 10.2% and the Greens are at 7.6% in the poll.  These numbers are middle-of-the-road for the Greens, but the shock result is that the NDP is falling so much from the 17.5% they won in the January election (and the 20% of so they were hovering at in the months after the election).  Two other recent Decima polls (here and here) have the NDP at 12%.  It seems like they have lost about 1/3 of their support.

So, what impact are the Greens having on these NDP results?  I've made the argument in the past that the GPC is drawing it's support from across the political spectrum.  I think that opinion is reinforced by comments like the following in this article by Decima pollster Bruce Anderson:

"The Greens are biting from everybody," said Anderson.

And here are some quotes from an article from back in August that shows that the NDP themselves know that the Greens aren't the root cause of their problems:

Brad Lavigne, communications director for the NDP, says it may be oversimplifying to cast the Greens, New Democrats and Liberals as one, large, left-wing mass. The NDP, more than other parties, has done some intense study of the Greens' vote and has found that people who cast their ballots that way could just as likely vote Conservative. And sometimes, the Green party vote is just citizens' way of saying "none of the above."

"If the Green party disappeared tomorrow, it might realistically mean an extra 1 per cent for the NDP, an extra 1 per cent for the Liberals and, yes, an extra 1 per cent for the Conservatives," Lavigne says.

Granted, this article shows that NDP voters are more likely to consider jumping to the Greens.  But the point is is that people should take any current and future NDP complaints about the Greens and vote-splitting - like the ones in this article - with a grain of salt.  The NDP is losing votes to everyone, not just the Greens.  They may want to consider taking more concrete action on the proportional representation initiatives they've been promising, while they have the influence to do so.

Some additional analysis on the problems the NDP is having is here and here.