Any bros advise whether there is a sim card available in   Hong Kon that also covers use in  Vietnam and Macau please. This avoids me having to buy a new sim card in each country.

I believe China mobile have roaming all over Asia areas. You might want to go to their store and ask. I think it's around $250 hkd a month.
I have China mobile that im using while staying in HK. But when I'm in China, I can only answer calls and sent text messages.
Hope this help.

I think Issue151 was asking a different question.  If I read the OP correctly, he doesn't want a monthly subscription (which can roam), he wants a pay-as-you-go SIM.

I have yet to find a SIM on a pay as you go plan that allows me to roam.  I would certainly be a buyer of such a thing if the brethren here know of one.  I'm just getting too damn many hobby sims!

Yes, I am looking for a pay and go number. Guess it will have to be seperate sim cards then. Thanks

no... I think most of the prepaid cards can go roaming.. you just need sufficient credit in the card.. some country's
require you to contact their operators to confirm that the sim has international roaming..

i might be wrong.. but I have using it for several countries..

example Singapore/ thailand indonesia and some other parts of asia..

Be careful for Vietnam, I went there with my HK China Mobile prepaid sim card (roaming fine in China and Macau), but couldn't work in Vietnam. So I had to buy a sim card there. You can find some everywhere, and note that some prepaid simcards will need you to give your passport details (the sim card will stay active 2 years if I remember correctly), while some don't need registration (valid only 6 months, then you need to register).

amazon.com has international sim cards...some cover 200 countries. Buying this way you can get the card before the trip

I know globe sim from PI can roam to china and the us. Other countries as well :p. Have to send a text to the service to enable roaming before you leave.

Let me talk about sim cards...

- There is NO global or even multi-country wireless provider.  Each country's laws about foreign ownership and how wireless infrastructure works prevents that.  As an example, Virgin Mobile is based in the United Kingdom but you've also seen the Virgin brand in America and Canada and so you think that your UK Virgin sim can work in America.  WRONG.  Actually it's local carriers that either partnered with Virgin UK or they have the rights to use the Virgin brand.  Either way, you're actually using the local carriers.

- Hong Kong's 3 network allows you to use their prepaid sims in a small handful of other Asian countries.  All that means is that 3 has a roaming agreement with carriers in Thailand, Philippines or whatever country it is.  With a 3 sim you're not using 3 in Thailand, you're roaming on the DTAC (or whatever company) network.  Roaming charges apply.

- As per above, whichever wireless carrier you're with, when you go traveling and use the same sim, you'll just be roaming on a local carrier's network and paying the roaming charges as per your home carrier's agreement.  The reason people recommend travelers buying a local sim when traveling is because roaming charges are often 10x more expensive than your regular at home local charges.

- Tri- and qua-band phones are often recommended because the GSM infrastructure is widespread around the world, allowing you to roam on that country's wireless carriers.  Quad-band phones were unique maybe 5 years ago, but since chipsets have gone down in price pretty much every phone is tri- or quad-band. CDMA was a former competing protocol against GSM but carriers in major countries are on the way to converting both CDMA and GSM to LTE.  There is also the AWS/1700 protocol which is used by some wireless carriers.  If you have an AWS phone then you're screwed.  You'll need to buy a new phone to use in other places.

what about three?  I know three works in most asian countries, not sure about viet

Firstly, as Freelancer explained, each mobile carrier is independent. There are 3's around the globe, but each one has to operate as an indendant company.

That being said, it is not wide-spread knowledge that there are indeed multi-country SIM cards. These SIM cards tend to be more expensive than country specific ones. Here is an example of one.
http://www.cellularabroad.com/cellAbroadSIMcard.php