It is a complicated business to purchase a domain name that is registered to someone else. Our support is strictly limited to the following advice.
If the domain is due to expire, that means nothing, the owner of the domain will likely renew their domain. If you really want the domain, contact the owner and make an offer to purchase.
If the domain has expired, then it may still be locked for a short time. This is to allow the owner a chance to renew their domain, even though they may have forgotten to renew due to out of date contact details.
The grace peiod In NZ, is 90 days, and depending on the domain suffix, it can be as little as 2 weeks or as much as 90 days. Sometimes, the owners registry will take ownership of the domain on expiry, then sell the domain for much more than the renewal price. They typically do this to get money out of the old owner, who forgot to renew, but took longer than the grace period. Prices are normally in the $150 zone... However, if the old owner doesn't renew, then they may sell the domain via auction in the Domain After Market...
People other than original owner and original registry may also be watching a domain for expiry. Many people have robot scripts setup to automatically register these domains as they become available, and they will beat anyone who trys to do the same thing as a human.
Take your chances if you like on an expired domain, but you'd probably have more luck offering the owner $100 to secure your chances. This would require you contact the owner as above.
We cannot provide any support for you in this process. Some IP lawyers specialise in this task, and can charge many thousands to help you. We think that would be wasted money, and better you offer the current owner that money to sweeten the deal. But beware dealing with other people off shore. Buyer beware of fraud.