depending on the AP you should be able to use 11a and 11b/g at the same time, most have separate radios for a and b/g. I know with my unit I normally use 11a around home and autoroam onto 11b as I get further away (requires a a/b pc card as well).

11b and 11g will work together, but you will have to make a comprimise on either the b or g throughput. only 1 11b device associated or even near a 11g AP will cause this.

11g users a different preamble to 11b and as such 11b traffic can't see 11g traffic without help. 11g will wait if 11b is transmitting, 11b wont wait. A collision occurs and both streams back off after completely trasmittiing the packets. 11g has a shorter contention back off than 11b and so will often get talked ove the top of by 11b after a collision. In this scenario 11g traffic really suffers.

To remidy this you need to turn "protection" on on the AP. This then makes the AP reserve the medium using a 11b PHY CTS to Self command. This gives 11g the medium by flowcontrolling the 11b traffic. Problem is now 11g gets lots of bandwidth, the 11b traffic comprimsed. Also as 11g traffic has to send a 11b CTS comaand this is done at the "LOWER" 11b data rates (often 1Mbps) so comprising all traffic by chewing up bandwitdth for control messages.

Basically either have all 11g devices, keep 11b devices well away or use 11a

Just bear in mind bonding of 11a amd 11b channels to get 108MB is a FCC standard only and as such only applies to North America. NZ is alligned with ETSI standards out of europe and channel bonding is not allowed. Also ETSI uses slightly different frequencies to FCC

For 11a
FCC uses 5.15-5.25Ghz, 5.25-5.35Ghz and 5.725-5.825Ghz.

Etsi uses 5.15-5.25Ghz, 5.25-5.35Ghz and 5.470-5.725Ghz.

Until the 802.11d standard (autosensing of country settings) comes out on APs and cards you will get potential for performance problems with mixing FCC and ETSI gear.. Most gear does allow you to set country settings to set it up for NZ
The 2.4 and 5 bands in NZ are unlicensed but not un regulated.