I have done quite a bit of research, and this is what I've found:
1. PUB's regulation only allows for indirect connection. ie. you are not supposed to connect a pump to 'suck' water directly out of PUB's mains. This is to prevent affecting your neighbour's water pressure everytime the pump kicks in. A system that meets PUB's criteria will consist of a water tank, a pump, a pressure tank, and the supporting electronics and pressure sensor to turn on and off the pump. A very reasonably priced system for small house holds that I've come across is the WILO CO/T all-in-one system, taking up a foot print of about 1m x 1m x 1m, costing 3-4k, excluding installation cost. The pump for this system runs on a 3-phase electrical supply, so your place will need to have this. Here's a contact to find out more about this setup:
Impetus Engineering Pte Ltd,
27 Defu Avenue 2,
Phone: +65 6280 0900,
Fax: +65 6282 3918
Sales contact: Mr. Philip Leong, Hp: +65 9693 4418
Just beware of the quotations from other contractors - some may proposed systems in excess of 10k, which might be totally an overkill for small households.
2. Due to cost, there are residents that connect pumps to the mains directly anyway. If the neighbours start to suffer from fluctuating water pressure, they may lodge complaints to PUB, and PUB will come looking for the pump in the neighbourhood. The smallest pump from Grundfos that is normally deployed in industrial applications sometimes gets used for this purpose: the MQ3-35. There are also people that brings in small pumps from M'sia, where is very common for households to install their own pumps to boost water pressure. Such deployment is against the regulation, and you'll stand the risks of being caught.
I hope this helps!