Tetrodotoxin (TTX) is known to be produced by several symbiotic Vibrio bacteria found in Pufferfish, Blue-ringed octopus, Newts and other species. Many species are believed to have developed immunities to the toxin through natural selection, and use it as a defense mechanism. The toxin is what is in pufferfish but pufferfish may not be directly responsible, bacteria could be spreading and infecting the dolphins, penguis and other sea life.
TTX is about 100 times more toxic than cyanide.
The TTX found in the sea slugs is possibly produced by a bacterial infection in the slug which is the slugs, and the sea slugs bodies are washing up on the beach, sea slugs are known to annually wash up on the beach when they die when they get old. Scientists have only confirmed that the sea slugs are toxic with high levels of TTX, washing up on the beach, and dogs have bitten or eaten the slugs and died from TTX.
TTX is related most commonly to pufferfish but this does not mean pufferfish are the culprits, the media just uses the puffer fish as a tool to explain what the toxin is to the average Joe, if they just said “Tetrodotoxin” everyone would just say “what the is that?”; Japanese scientists showed that species of pufferfish that are known to be toxic, when bread in aquariums do not become toxic as they do not have the vibrio bacteria which is assumed to be picked up from the environment at some point in the pufferfish”s life, and those species that produce TTX are known to excrete large quantities of the toxin from its skin as one of their defence mechanisms to kill predators.
Research is not spontaneous, and toxin testing is nothing like TV would suggest, you do not send a sample to a lab and say “what killed it” and have some guy come back six minutes later with the exact toxin, the concentration and the source it came from; you have to test for each toxin individually, which can take days or even weeks, and testing can be very expensive. Most toxin testing is done by Liquid Chromatography- Mass Spectrometry (LCMS), Gas Chromatography (GC), Mouse Bioassay, or Enzyme Immuno Assay (ELISA), in the case of Mouse Bioassays you can confirm that there is a toxin, but due to the non-specific nature of the testing will not give accurate information on what the toxin is, methods such as LCMS and ELISA need to be setup specifically for each determinant, and takes months or even years of research to setup.
Suggesting that “they” (I presume you mean the big corporation in the show “The X-Files” which conspired to undermine the human race by handing us all over the aliens which abducted us for perverted anal probe experiments) are hiding any information implies you believe they actually know any more than we do; which is absolutely absurd. “They” (by “they” I mean the evil conglomerate of sensible people) invented the term “conspiracy nut” for a reason.
If you are one of the many suggesting Sodium fluoroacetate (1080), Sulphur from volcanic activity, among other crazy ideas “conspiracy nuts” have come up with to blame DOC for, claiming there will be a big hush up. 1080 isn’t likely to cause death to marine life due to the vast ocean that it would have to be dumped into, and the insane levels it would have to be put in the water at to cause an effect on the marine life would be insurmountable excessively expensive, and pointless. 1080 decomposes relatively quickly in water, is highly soluble in water, and even if sprayed in the area (which “they” (DOC bureaucrats covering their arses) claim hasn’t been sprayed in that particular area in years) would not be in a large enough dose. Don’t get me wrong, I hate the idea that 1080 is being sprayed over our country, and would highly recommend everyone swamping their MP with letters of their concern about the compound, but I just don’t see how this could cause this sort of impact (unless perhaps, a drunk aviator flying with a faulty latch holding a 1080 canister which was attached to the plane by an avid fisher who still had slimy hands after handing their extra potent fish bait, over the area, and the canister fell off the plane, and the canister fell into the water, was somehow buoyant enough to not immediately sink and slowly leaked the toxin over the dolphins, birds and penguins attracted to the canister, killing them all, the canister then sunk to the bottom of the ocean never to be found again, and after the aviator landed noticing he had no canister began to suspect he forgot to put one on the plane before takeoff and in order to cover up his blunder falsified paperwork in order to prevent an investigation which would ultimately lead to finding he was intoxicated whilst flying and resulting in his license being revoked and his employment terminated).
You should stop watching stupid television shows like CSI and actually find out what the real world is like, and how things are done, if you are too lazy to go into the real world to do your research, then I recommend Wikipedia, coupled with Google Scholar. Expecting information on every little detail this early on is unrealistic, however it is not unrealistic to question why it took so long for samples to be taken and tested to begin with, blame bureaucracy I guess.
As far as I am aware not everything is known about what is going on, samples from dolphins, penguins and fish have not even been tested to confirm or deny whether they were from TTX, they may or may not be related but we will not know until the testing has been completed and reported.
Scientists are currently only saying that TTX is what caused the DOG deaths, due to the very high levels of TTX present in the dead sea slugs which washed up on the beach.
Most of this health risk also appears more widespread than it is, many dog owners reporting their dog is sick, hours or days after being near the beach; these cases are unlikely due to TTX as TTX would normally just kill the dogs within minutes, the dogs would get sick, throw up, then die if they were exposed to TTX. The dogs that just got sick are likely just catching a virus, people get sick, animals get sick too, it is normal and owners would panic and call the health lines which would just add red herring statistics to the whole event.
TTX has not been reported in New Zealand before that I am aware of, this however does not mean it has never been here, but this is likely the first time it has grown to a level which is deemed to be a health risk.
With the high levels of nutrients in the waters due to increase farming, warmer climates in the area, and many other environmental changes that have naturally occur, Vibro bacteria which produces the toxin may be spreading in the area and killing the dolphins and other species, but we will not know until all testing is completed.
It is absolutely amazing that the scientists at Cawthron found out what toxin was present that killed the dogs this quickly, let alone even at all, most toxic events around the world never even result in the toxin ever being determined.

Sources:
http://en.wikipedia.org/wiki/Tetraodontidae
http://en.wikipedia.org/wiki/Tetrodotoxin
http://en.wikipedia.org/wiki/Vibrio
http://en.wikipedia.org/wiki/ELISA
http://en.wikipedia.org/wiki/Liquid_chromatography-mass_spectrometry
http://en.wikipedia.org/wiki/Sodium_fluoroacetate
http://www.contest.co.nz/general-chat/30950-dogs-dying-north-shore-beaches-urgent-notice-dont-walk-nth-shore-beaches-2.html
http://www.seaslugforum.net/