BuzzyBee, there are two distinct categories of skills with regards to communication -- speech and language. Speech is the production of sound, and language is the meaningful use of interactive communications. Your concerns might be about both of these issues, or just one. It's helpful to define which is your primary concern before investigating further.
There is a sequential development of speech sounds that most children follow, more or less in a similar pattern. Between the age of 1.5 and 3, they usually master p, m, h, n, and w sounds. Next comes b (between 1.5 and 4 years), then k, g, and d (between 2 and 4 years). t and ng usually come between 2 and 6 years, and f and y come between 2.5 and 4 years. Other sounds don't usually come until later -- r, l, s, ch and sh, z, j, v, th, and zh are all usually mastered after the age of 3. It might help you to go through the alphabet and note down which speech sounds your child can produce. If that's a bit difficult, try writing down all of the words your child says in a day -- that will show you what speech sounds your child is producing in a different way, and also give you an indication of their vocabulary. It's also a useful diagnostic tool for speech therapists, should you need to visit one.
Hearing difficulties can contribute significantly to delays in speech and language development. My daughter, who will be 3 next month, had diminished hearing in both ears due to recurrent ear infections and glue ear. This contributed to a delay in her speech. When she had grommets at the age of 18 months, we saw almost immediate significant improvement in her speech and language as well as in her listening skills. At "almost 3", she still doesn't say k or g sounds, but that's okay because developmentally, she's got another year to master those sounds before we start worrying about it.
If you're concerned about your child's speech or language, your best bet is to talk to your child's Plunket nurse at their 24-month Well Child visit. If you can't wait until then, go to your GP and raise your concerns. They can best help you if you take a little bit of time to make some notes on your specific concerns before you go. Such as, "My child doesn't seem to hear me until I am right in front of their face", "My child only says these 10 words and no others", "My child gets frustrated when trying to communicate with me, and uses gestures and grunts instead of words"... That sort of thing. Try to cast your mind back over whether they have had any colds or ear infections that the GP/Plunket nurse doesn't know about. Just create a basic picture of the issues surrounding your child's speech and language development so that it will be clearer to the experts what to look at.
Best of luck! I know how frustrating it can be when you're not sure if you should be worried or not, but I'm of the mindset that if you think there's a problem, it's better to get it checked out for your peace of mind. And as Jaycee said, they do things in their own time... Err on the side of caution if you're really worried, but try not to get too stressed out about it, and try not to compare your child to others too much as it'll drive you nuts. 
Edited by Katherine