Hi Merlion
I have just moved from Singapore (lah) to HK and we have brought our helper with us. TBH, she's currently here as a visitor as we didn't have time to get our own visas let alone hers. (The move was a quick one - only one month's notice).
The process is quite long and involved (much more so than in Singapore) and we used an agency in HK (who seem quite good) but who also recommended we use an agency in Singapore (who were dreadful - mainly because of communication problems. If you speak Mandarin, you'll be okay).
We now have our employment/dependant visas and will pop to Macau this weekend to validate them. After that, HK Immigration can start working on our helper's employment visa.
If you don't have time to sort the visa out before you get to Singapore, your helper can enter for 14 days as a visitor. But to save the constant journeys to Macau (and the risk she might be turned back by immigration), we went to the China Embassy on Tanglin Rd and got her a 60day visitor visa.
If you need more info, let me know.
cheers
Honkyblues
PS. To any HK immigration officers reading this, then obviously our helper is here purely as my companion and is not currently working.